feat: add runtimeconfig variable data source

[modular-magician]

What is this change?

This PR adds a data source for google_runtimeconfig_variable as requested on this issue.

Checklist

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). Comment here.
• Generated Terraform, and ran rake test and rake lint to ensure it passes unit and linter tests. - unrelated failure in products/appengine/api.yaml.
• 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_compute_new_datasource`

Derived from GoogleCloudPlatform/magic-modules#4475