Add vtctld get-routing-rules command

[maxenglander]

Adds a vtctld command to read live routing rules from the cluster:

pscale branch vtctld get-routing-rules <database> <branch>

This complements pscale branch routing-rules get, which reads from the schema snapshot.

Depends on planetscale/planetscale-go#313.

Manual testing

Tested end-to-end against a dev Vitess branch (org with direct_vtctld_access enabled) with the full stack deployed.

1. Created a Vitess database and waited for the default branch to become ready.
2. pscale branch vtctld get-routing-rules <database> <branch> returned {"rules":[]}.
3. Created a second keyspace on that branch:

   pscale keyspace create <database> <branch> target-ks --cluster-size PS-10 --wait
   

4. Created an identical customers table on both the default keyspace and target-ks.
5. Applied routing rules so rdonly queries against customers on the default keyspace are served from target-ks:

   {
     "rules": [
       {"from_table": "customers@rdonly", "to_tables": ["target-ks.customers"]},
       {"from_table": "target-ks.customers", "to_tables": ["<default-keyspace>.customers"]},
       {"from_table": "customers", "to_tables": ["<default-keyspace>.customers"]}
     ]
   }

   pscale branch routing-rules update <database> <branch> --routing-rules routing-rules.json
   

6. pscale branch vtctld get-routing-rules <database> <branch> returned:

   {
     "rules": [
       {
         "fromTable": "customers@rdonly",
         "toTables": ["target-ks.customers"]
       },
       {
         "fromTable": "target-ks.customers",
         "toTables": ["<default-keyspace>.customers"]
       },
       {
         "fromTable": "customers",
         "toTables": ["<default-keyspace>.customers"]
       }
     ]
   }