CODEOWNERS Cleanup: Restrict codeowners enforcement skipping to specified emails

The `verify-codeowners.yml` pipeline template should allow skipping codeowners verification for the same reasons as `create-apireview.yml`, but restricted to an approved set of email addresses rather than allowing anyone to bypass it via `Skip.VerifyCodeowners`.

## Scope

- Add an evaluation step that sets a single pipeline variable (`ShouldSkipVerifyCodeowners`) which the later verification steps gate on.
- Only skip when `Skip.VerifyCodeowners` is `true` **and** the requesting email (`Build.RequestedForEmail`) is in an allowed set.
- Allowed set = a default list of emails plus additional emails supplied via a new `AllowSkipEmails` template parameter.

Part of the CODEOWNERS cleanup work (#14754).

Start with the verify-codeowners.yml pipeline template and compare its skip behavior with create-apireview.yml. Trace the later verification steps and confirm they use the single ShouldSkipVerifyCodeowners variable. Done means skipping occurs only for true Skip.VerifyCodeowners requests from the default or AllowSkipEmails set, while other requests still verify CODEOWNERS.

Written by the indexing model from the issue text.