New data source: okta_app_group_assignments

[ymylei]

Creates a new data source, intended to replace the groups data on the okta_app_* resources.

Initial implementation only provides the list of groups IDs, much like the groups interface does.

[ymylei]

Acceptance test results:

<pruned>

[ymylei]

Updated test results:

➜  terraform-provider-okta git:(data_okta_group_assignments) TEST_FILTER="TestAccOktaDataSourceAppGroupAssignments_read" make testacc
go mod tidy
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v  -run TestAccOktaDataSourceAppGroupAssignments_read -timeout 120m
?       github.com/okta/terraform-provider-okta [no test files]
=== RUN   TestAccOktaDataSourceAppGroupAssignments_read
--- PASS: TestAccOktaDataSourceAppGroupAssignments_read (12.02s)
PASS
ok      github.com/okta/terraform-provider-okta/okta    12.247s
?       github.com/okta/terraform-provider-okta/sdk     [no test files]