Add the google_dataflow_flex_template_job resource

[rileykarson]

Supersedes #3520, fixes hashicorp/terraform-provider-google#6656

I squashed the original PR into a single commit and overlayed on top to keep the commit count manageable. I got this to a passing test, but spent most of my time getting that inline template right- the example linked from the docs was broken and there was some manual work with gcloud involved with getting a properly formatted template. I don't believe we need to document that part- it's already covered by Using Flex Templates.

I reviewed the original PR over a fairly long period of time and lost context several times throughout. A fresh look at most of the changes would be appreciated- most aren't mine, I made targeted changes to get this to a passing state.

Release Note Template for Downstream PRs (will be copied)

`google_dataflow_flex_template_job`

[google-cla]

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 89 insertions(+))
Terraform Beta: Diff ( 5 files changed, 407 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=134846"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccContainerCluster_withPrivateClusterConfigMissingCidrBlock|TestAccFolderIamAuditConfig_multiple|TestAccServiceAccountIamBinding_withAndWithoutCondition|TestAccServiceAccountIamMember_withAndWithoutCondition|TestAccMonitoringMetricDescriptor_monitoringMetricDescriptorAlertExample You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=134848"

[geojaz]

@googlebot I consent.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[c2thorn]

Tested it out. Minor nits, but otherwise LGTM.

[c2thorn]

Probably want to remove this?

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 88 insertions(+))
Terraform Beta: Diff ( 5 files changed, 405 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=135440"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccEndpointsService_basic|TestAccMonitoringMetricDescriptor_monitoringMetricDescriptorAlertExample You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=135443"