Restructure relay API messages

[adriansmares]

Summary

References #6721

This PR exists in order to make reviewing the PR above more easier to go through.

Changes

• Move the ServingRelayParameters.ForwardingLimits and ServingRelayParameters.UplinkForwardingRule to the top level of the API.

Testing

No additional testing is required. This is strictly restructuring the API, without any binary or JSON changes.

Regressions

N/A.

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.
• The steps/process to test this feature are clearly explained including testing for regressions.
• 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.