backend: Add service account impersonation to GCS Backend and update the docs

[alisdair]

0.14-targeted backport of #26700

Acceptance test output for the GCS backend

$ TF_ACC=1 go test -v ./backend/remote-state/gcs/
=== RUN   TestStateFile
=== PAUSE TestStateFile
=== RUN   TestRemoteClient
=== PAUSE TestRemoteClient
=== RUN   TestRemoteClientWithEncryption
=== PAUSE TestRemoteClientWithEncryption
=== RUN   TestRemoteLocks
=== PAUSE TestRemoteLocks
=== RUN   TestBackend
=== PAUSE TestBackend
=== RUN   TestBackendWithPrefix
=== PAUSE TestBackendWithPrefix
=== RUN   TestBackendWithEncryption
=== PAUSE TestBackendWithEncryption
=== CONT  TestStateFile
=== CONT  TestBackend
--- PASS: TestStateFile (0.00s)
=== CONT  TestRemoteLocks
=== CONT  TestRemoteClientWithEncryption
=== CONT  TestBackend
    backend_test.go:134: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f2478e1755e8-testbackend"), "encryption_key":cty.StringVal(""), "prefix":cty.StringVal("")}}
=== CONT  TestRemoteClientWithEncryption
    backend_test.go:80: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f2478e185ba0-testremoteclientwithencryption"), "encryption_key":cty.StringVal("yRyCOikXi1ZDNE0xN3yiFsJjg7LGimoLrGFcLZgQoVk="), "prefix":cty.StringVal("")}}
=== CONT  TestRemoteClient
=== CONT  TestRemoteLocks
    backend_test.go:100: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f2478e1784c8-testremotelocks"), "encryption_key":cty.StringVal(""), "prefix":cty.StringVal("")}}
=== CONT  TestRemoteClient
    backend_test.go:61: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f2478e1ec828-testremoteclient"), "encryption_key":cty.StringVal(""), "prefix":cty.StringVal("")}}
=== CONT  TestBackend
    backend_test.go:137: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f2478e1755e8-testbackend"), "encryption_key":cty.StringVal(""), "prefix":cty.StringVal("")}}
2020/11/06 09:42:42 Object deleted: default.tfstate
=== CONT  TestBackendWithEncryption
--- PASS: TestRemoteClientWithEncryption (6.22s)
=== CONT  TestBackendWithEncryption
    backend_test.go:163: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f2490109fb68-testbackendwithencryption"), "encryption_key":cty.StringVal("yRyCOikXi1ZDNE0xN3yiFsJjg7LGimoLrGFcLZgQoVk="), "prefix":cty.StringVal("")}}
--- PASS: TestRemoteClient (6.35s)
=== CONT  TestBackendWithPrefix
    backend_test.go:150: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f24908c8f020-testbackendwithprefix"), "encryption_key":cty.StringVal(""), "prefix":cty.StringVal("test/prefix")}}
--- PASS: TestRemoteLocks (6.59s)
=== CONT  TestBackendWithEncryption
    backend_test.go:166: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f2490109fb68-testbackendwithencryption"), "encryption_key":cty.StringVal("yRyCOikXi1ZDNE0xN3yiFsJjg7LGimoLrGFcLZgQoVk="), "prefix":cty.StringVal("")}}
=== CONT  TestBackendWithPrefix
    backend_test.go:153: TestBackendConfig on *gcs.Backend with configs.synthBody{Filename:"<TestWrapConfig>", Values:map[string]cty.Value{"bucket":cty.StringVal("tf-1644f24908c8f020-testbackendwithprefix"), "encryption_key":cty.StringVal(""), "prefix":cty.StringVal("test/prefix/")}}
=== CONT  TestBackend
    backend_test.go:140: TestBackend: testing state locking for *gcs.Backend
    backend_test.go:141: TestBackend: testing state locking for *gcs.Backend
=== CONT  TestBackendWithPrefix
    backend_test.go:156: TestBackend: testing state locking for *gcs.Backend
=== CONT  TestBackendWithEncryption
    backend_test.go:169: TestBackend: testing state locking for *gcs.Backend
2020/11/06 09:42:55 Object deleted: bar.tfstate
2020/11/06 09:42:55 Object deleted: default.tfstate
--- PASS: TestBackend (20.06s)
2020/11/06 09:42:56 Object deleted: test/prefix/bar.tfstate
2020/11/06 09:42:56 Object deleted: bar.tfstate
2020/11/06 09:42:56 Object deleted: test/prefix/default.tfstate
2020/11/06 09:42:57 Object deleted: default.tfstate
--- PASS: TestBackendWithPrefix (14.58s)
--- PASS: TestBackendWithEncryption (15.27s)
PASS
ok  	github.com/hashicorp/terraform/backend/remote-state/gcs	23.435s

[codecov]

Codecov Report

Merging #26837 into v0.14 will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Impacted Files	Coverage Δ
backend/remote-state/gcs/backend.go	0.00% <0.00%> (ø)
dag/marshal.go	53.42% <0.00%> (-1.37%)	⬇️
terraform/evaluate.go	53.11% <0.00%> (+0.41%)	⬆️
terraform/eval_apply.go	73.45% <0.00%> (+0.58%)	⬆️
internal/providercache/dir.go	71.42% <0.00%> (+6.12%)	⬆️

[ghost]

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