feat: add data_source_google_service_account_jwt

[modular-magician]

Closes #6686.

This PR adds a google_service_account_jwt data source, similar to the google_service_account_id_token data source. The difference between this and the id_token resource is that the jwt data source can produce an arbitrary JWT, e.g. with a custom sub field, etc.

We're interested in this in order to use our existing Google Cloud credentials to authenticate with other terraform providers that can trust JWTs signed by Google. The linked issue is for Vault, which is not our use-case, but would make sense as well.

Note: I've contributed to terraform-provider-google before, but this is my first time contributing to magic-modules! It definitely feels a little trickier, so let me know if you'd like anything else from me. I would like to add tests to this, but I wanted to at least get a temperature check on the PR before investing more time in it.

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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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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).
• Ran relevant acceptance tests (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_service_account_jwt`

Derived from GoogleCloudPlatform/magic-modules#6239

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