API: ks_tablets

[shlomi-noach]

This PR adds an API endpoint to vtctld:

• /api/ks_tables/my_keyspace
• /api/ks_tables/my_keyspace/-80

The API returns a list of all tablets for a given keyspace, optionally further filtered by shard.

The context for this PR is https://github.com/github/freno, and in particular github/freno#70: giving the freno throttling tool an easy way to determine the roster of MySQL servers (host & port) it needs to poll for replication lag.

cc @github/database-infrastructure

[sougou]

You need to sign your commits :)

[shlomi-noach]

You need to sign your commits :)

The steps illustrated in https://github.com/vitessio/vitess/pull/4207/checks?check_run_id=18762836 caused many more commits to be signed by me, which were not, in fact, my commits. This leads to a merge conflict now.

I'm likely to close this PR and open a new one.

[shlomi-noach]

Superseded by #4232, and I will have local cleanup to do.