[vtctld] Add GetSrvVSchema command

[ajm188]

Description

This implements GetSrvVSchema in the vtctld service. Pretty straightforward.

Sample output from local example:

❯ vtctldclient --server "localhost:15999" GetSrvVSchema zone1
{
  "keyspaces": {
    "commerce": {
      "sharded": false,
      "vindexes": {
      },
      "tables": {
        "corder": {
          "type": "",
          "column_vindexes": [
          ],
          "auto_increment": null,
          "columns": [
          ],
          "pinned": "",
          "column_list_authoritative": false
        },
        "customer": {
          "type": "",
          "column_vindexes": [
          ],
          "auto_increment": null,
          "columns": [
          ],
          "pinned": "",
          "column_list_authoritative": false
        },
        "product": {
          "type": "",
          "column_vindexes": [
          ],
          "auto_increment": null,
          "columns": [
          ],
          "pinned": "",
          "column_list_authoritative": false
        }
      },
      "require_explicit_routing": false
    }
  },
  "routing_rules": {
    "rules": [
    ]
  }
}

Related Issue(s)

• Migrate simple read-only schema commands to vtctld gRPC API #7134

Checklist

• Should this PR be backported? No
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build
• VTAdmin

[doeg]

A task so straightforward I could have done it!!!!!

(Thank you! 💛 )

[doeg]

So amazing to have tests 😭 and the tests are good