Throttler: re-implement 'ALTER VITESS_MIGRATION ... THROTTLE ...' to write to 'topo'

[shlomi-noach]

Context: #13341

Starting with #13351, we store app-throttling rules in topo. This means vtctldclient UpdateThrottlerConfig now supports per-app throttling configuration, e.g.:

$ vtctldclient UpdateThrottlerConfig --throttle-app="vreplication" --throttle-app-ratio=0.5 --throttle-app-duration="30m" commerce

However, we also already have existing ALTER VITESS_MIGRATION ... (THROTTLE|UNTHROTTLE) ... set of commands. Today, these are sent to the backend tablets just like any other ALTER VITESS_MIGRATION command such as CANCEL, COMPLETE, etc.

As a reminder, ALTER VITESS_MIGRATION ... (THROTTLE|UNTHROTTLE) ... was constructed for a convenient control over schema migrations. . It applies either to a given UUID or to "online-ddl" apps. Examples:

alter vitess_migration '96ace339_23b6_11ee_b8b7_0a43f95f28a3' throttle expire `1h`;
alter vitess_migration '96ace339_23b6_11ee_b8b7_0a43f95f28a3' unthrottle;
alter vitess_migration throttle all;
alter vitess_migration unthrottle all;

The command must now reroute through topo: it must not reach the backend tablets directly. It should apply to Keyspace and SrvKeyspace in topo, on which the tablet throttler listens.