Add datasource for google_compute_instance_group_manager

[kutysam]

Google compute instance group manager and region instance group manager datasources seem to be missing in google's terraform provider.

We need datasource to extract things like instance_template from the instance group manager. There are two versions, region and zone. Both are very similar.

The APIs that these 2 PRs are related to are

1. google_compute_instance_group_manager - https://cloud.google.com/compute/docs/reference/rest/v1/instanceGroupManagers/get
2. google_compute_region_instance_group_manager - https://cloud.google.com/compute/docs/reference/rest/v1/instanceRegionGroupManagers/get

If this PR is for Terraform, I acknowledge that I have:

• [X] Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• [X] Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• [x] Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• [X] Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• [X] Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

data_source_google_compute_instance_group_manager: Add datasource for google compute instance group manager
data_source_google_compute_region_instance_group_manager: Add datasource for google compute region instance group manager

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[modular-magician]

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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 ( 5 files changed, 698 insertions(+)) Terraform Beta: Diff ( 5 files changed, 698 insertions(+)) TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

The provider crashed while running the VCR tests in REPLAYING mode Please fix it to complete your PR View the build log

[kutysam]

Rather than handwriting the datasource, can you use the DS utility functions? That will keep the schema in sync. This an an example: https://github.com/GoogleCloudPlatform/magic-modules/blob/main/mmv1/third_party/terraform/data_sources/data_source_google_compute_disk.go

Thanks for this! I'll work on this format in coming days.

[kutysam]

Closing this PR