ringhash: allow setting request hash key explicitly

[atollena]

Implement part 1 of A76: the ability to set the request hash key, to extract the hash from a header. This allows using ring hash without xDS.

RELEASE NOTES:

• ringhash: Allow setting request hash key explicitly using request_metadata_key load balancing config field (gRFC A76).

[atollena]

Ready to review, will wait for A76 to be merged before merging this.

[arvindbr8]

Sure, will take a look.

[github-actions[bot]]

This PR is labeled as requiring an update from the reporter, and no update has been received after 6 days. If no update is provided in the next 7 days, this issue will be automatically closed.

[atollena]

You can hold on reviewing while I investigate making pick first the leaf policy for ring hash, as suggested in https://github.com/grpc/proposal/pull/412#discussion_r1591438225.