feat: add data_source_google_service_account_jwt

[modular-magician]

Closes hashicorp/terraform-provider-google#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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Release Note Template for Downstream PRs (will be copied)

`google_service_account_jwt`

Derived from GoogleCloudPlatform/magic-modules#6239