Add custom logic for handling `_Default` and `_Required` for `google_logging_project_sink`

[ichbinfrog]

This PR adds logic to handle _Default and _Required singleton log sinks for google_logging_project_sink.

The logic is similar to google_logging_project_bucket_config where terraform would import those sinks on creation and remove from the state on deletion. Automatic imports of this resource is particularly helpful in a couple of use cases such as:

• Handling log regionality: for an organization with a lot of projects, manual importing of the log routers to route a specific log bucket is not feasible.

This PR is heavily inspired by https://github.com/hashicorp/terraform-provider-google/pull/9903/files but adapts it to the magic-modules repo and reduces the scope for simpler deployment. There's also an additional complexity to handle a case where these buckets are removed by using the new organization default settings.

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• [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.
• [ ] 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).
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Release Note Template for Downstream PRs (will be copied)

logging: improve handling or `_Default` and `_Required` for `google_logging_project_sink`