[vtctldserver] Migrate remaining ServingGraph rpcs to VtctldServer

[ajm188]

Description

This PR migrates the following RPCs to the VtctldServer:

• DeleteSrvVSchema
• GetSrvKeyspaceNames

Usage

Everything on the local example.

❯ vtctldclient --server "localhost:15999" GetSrvKeyspaceNames
{
  "zone1": {
    "names": [
      "commerce"
    ]
  }
}

After running through the 2xx examples:

❯ vtctldclient --server "localhost:15999" GetSrvKeyspaceNames
{
  "zone1": {
    "names": [
      "commerce",
      "customer"
    ]
  }
}
❯ vtctldclient --server "localhost:15999" GetSrvVSchemas
{
  "zone1": {
    "keyspaces": {
      "commerce": {
        "tables": {
          "product": {}
        }
      },
      "customer": {
        "tables": {
          "corder": {},
          "customer": {}
        }
      }
    },
    "routing_rules": {}
  }
}
❯ vtctldclient --server "localhost:15999" DeleteSrvVSchema zone1
❯ vtctlclient -server "localhost:15999" TopoCat -cell zone1 -decode_proto_json SrvVSchema
TopoCat: Get(SrvVSchema) failed: node doesn't exist: /vitess/zone1/SrvVSchema
null
TopoCat Error: rpc error: code = Unknown desc = TopoCat: some paths had errors
E0602 17:39:16.972427   81449 main.go:72] remote error: rpc error: code = Unknown desc = TopoCat: some paths had errors

Related Issue(s)

• Closes Migrate remaining ServingGraph commands to VtctldServer #8230

Checklist

• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[rafael]

This implementation seems to diverge from the existent one in vtctl: https://github.com/tinyspeck/vitess/blob/22bdd8a9304ec2499414a2526ea644a2925259d5/go/vt/vtctl/vtctl.go#L3240

Curious of the rationale for this

[ajm188]

In VTAdmin we've been finding that we almost always want to get "all of the things" for a given resource, so this let's us make one call to a vtctld rather than one call to get all the cells and then one call per cell.

Which reminds me ... I meant to update the old implementation to call this one!

[ajm188]

Just to demonstrate the old commands still work after my latest commit:

❯ vtctlclient -server "localhost:15999" GetSrvKeyspaceNames zone1
commerce

[ajm188]

rebased and regenerated protos

[doeg]

This looks good to me, and the new GetSrvKeyspaceNames behaviour seems like a helpful default. You might want another +1 from @rafael or @deepthi, since this is ✨ new ✨

[deepthi]

LGTM