Introduce google_firestore_field resource.

[modular-magician]

Introduces a single resource for managing Firestore fields, matching the REST API.

Introduces 2 primary divergences from the proto/REST API:

1. Firestore fields do not support deletion (this is because all fields
   implicitly can exist in a schemaless system). Deletion of a terraform
   resource will clear any "overrides" for this field (TTL or indexing).
2. Single field indexes in the proto API were overly nested to allow
   reusing the same resource for composite indexes. The terraform
   resource unnests the 1-length field in the indexes list.

fixes #12151
fixes #11419
fixes #7593

Tested with make testacc TEST=./google TESTARGS='-run=TestAccFirestoreField'

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

• 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).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

`google_firestore_field`

Derived from GoogleCloudPlatform/magic-modules#7853

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