Update google_compute_router_interface import, acctest, docs

[modular-magician]

Fixes #14331

This PR:

• Adds a 4-part import format for the google_compute_router_interface resource to allow passing in project information.
• Updates an existing acceptance test to test import with the 4-part import id
• Update the docs

The resource is handwritten.

Router interfaces are not independent entities in the Compute API, they're info attached to Router entities. This means they don't have self links/GET-able URIs specific to the interface that the resource represents, and instead the provider makes up a {{region}}/{{router}}/{{name}} identifier for the interface. As this lacks project info the (old) import functionality relies on project being set in the provider configuration/ENVs.

I've left the id of the resource as the old 3-part identifier as I didn't want to introduce breaking changes.

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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).
• 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)

compute: fixed an import bug for `google_compute_router_interface` that happened when project was not set in the provider configuration or via environment variable

Derived from GoogleCloudPlatform/magic-modules#7758

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