Update DownlinkQueue operations RateLimit keys

[nicholaspcr]

Summary

Backport of https://github.com/TheThingsIndustries/lorawan-stack/pull/4787

Changes

• Add implementation of the Keyer interface for DownlinkQueueRequest
  • This enforces rate-limiting on a application ID basis.

Regressions

This changes the limiting factor of operations related to downlinks in the Application Server, it now is much easier to get rate limited.

Notes for Reviewers

This is a backport PR, therefore I will merge this after the CI passes.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.