Summary:
Add in hooks to drop the xcluster ddl replication extension on replication deletion / alter
replication remove namespace.

On the target side we always delete the extension since we do not support daisy chaining or N:1
replication.
On the source side, we first verify that no other automatic-mode outbound replication groups include
this namespace, then do the deletion.
Jira: DB-7987

Test Plan:
```
ybd --cxx-test xcluster_ddl_replication-test --gtest_filter "XClusterDDLReplicationTest.BasicSetupAlterTeardown"
```

Reviewers: hsunder, xCluster

Reviewed By: hsunder

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D40628