feat(app-check): Debug token support for the activate method

[furkankurt]

Description

This feature enables developers to use a pre-generated debug token rather than generating one at runtime. It is particularly beneficial in CI environments where manually copying the token from logs and pasting it into the Firebase Console is not feasible.
By using a pre-generated token, the setup becomes more automated and less error-prone, streamlining the development and testing process.
This PR is an improved version of #13101. Since the maintainer doesn't follow his PR, I wanted to improve it and open another PR. The improvements include better spelling and an added documentation section for how to add a debug token.

Related Issues

Fixes #11719

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.