Create a rubocop to enforce prefixing authentication tokens

In https://docs.gitlab.com/ee/development/secure_coding_guidelines.html#token-prefixes
we mandate that tokens are prefixed, so that they can be detected by
secret detection if leaked. Now that many of our tokens have prefixes
we should enforce this for new additions.

Proactively including a prefix increases efficiency: developers don't
need to go back and add a prefix later, there's no worries about
needing to rotate existing tokens later, no need for feature flagged
rollout, etc.

The rubocop has been disabled for existing violations with either a
wontfix comment, or a TODO with issue URL comment. The rationale for
these decisions is at
https://gitlab.com/gitlab-org/gitlab/-/issues/439007#note_1740969806

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/439007