Add endpoint to request recovery point for remote shard

[timvisee]

Tracked in: #3477

This adds an gRPC (internal) interface to request the recovery point of a remote shard. It also adds a convenience function to the RemoteShard to easily access this.

It has a new RecoveryPoint type, similar to the ClockMap, to hold the clock states. Right now it just returns the last seen clock states as recovery point.

All Submissions:

• Contributions should target the dev branch. Did you create your branch from dev?
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests?
2. Have you formatted your code locally using cargo +nightly fmt --all command prior to submission?
3. Have you checked your code using cargo clippy --all --all-features command?

[timvisee]

It seems like our gRPC generator is having a stroke, moving a whole lot of things around.