Addresses #11864.

Description

Make some improvements to routing for reference tables:

• Support for globally routing reference tables that are identically named with their source.
• Optimize joining so that reference tables are joined with the optimal keyspace.
• Redirect DML for reference tables back to their source.

Given:

• An unsharded keyspace k1 and an sharded keyspace k2.
• Source table k1.r and reference table k2.r.
• Unsharded table k1.a
• Sharded table k2.b.

And these queries:

1. SELECT k2.b JOIN k1.r
2. SELECT a JOIN r
3. SELECT b JOIN r
4. INSERT INTO r

The first query will succeed but will issue queries to both k1 and k2, even though the query could be satisifed just by routing to k2.

The next three queries will fail with

This PR adds limited support for optimally joining and globally routing these three queries:

1. The first query is routed exclusively to k2.
   2 The second query will join k1.a to k1.r.

• The third query will join k2.b to k2.r.
• The fourth query will be routed to the unsharded source table r.

Details

Some notes about the implementation:

• Have intentionally put a lot of constraints (more on this in docs) on this feature. For example requiring that a source is unique in a given keyspace. Future PRs could relax these constraints, but figured it's better to start with a smaller initially.
• This feature will override qualified queries. E.g. SELECT k2.a JOIN k1.r will route r to k2, even though the user has explicitly requested k1.r.
• Routing rules take precedence over this feature.

Checklist

• Add support for source to table VSchema definition.
• Treat reference tables and their source as globally unambiguous.
• Route unqualified INSERT to reference tables back to the source.
• Join unqualified table to the keyspace-local reference.
• Write tests
• Add docs