[vtctldserver] Migrate CellInfo, CellAlias rw RPCs

[ajm188]

Description

This migrates the read-write cell and cell alias vtctl commands to the grpc vtctldserver, namely:

• AddCellInfo
• AddCellsAlias
• DeleteCellInfo
• DeleteCellsAlias
• UpdateCellInfo
• UpdateCellsAlias

Note, there is an underlying issue in the (*topo.Server).DeleteCellInfo method, which I'm filing a bug for, and will link back here when I do.

Example usage

❯ vtctldclient --server "localhost:15999" GetCellInfo zone1
{
  "server_address": "127.0.0.1:2379",
  "root": "/vitess/zone1"
}
❯ vtctldclient --server "localhost:15999" AddCellInfo -r "/vitess/zone2" zone2
Created cell: zone2
❯ vtctldclient --server "localhost:15999" GetCellInfo zone2
{
  "server_address": "",
  "root": "/vitess/zone2"
}
❯ vtctldclient --server "localhost:15999" UpdateCellInfo zone2 -a "127.0.0.1:19999"
Updated cell zone2. New CellInfo:
{
  "server_address": "127.0.0.1:19999",
  "root": "/vitess/zone2"
}
❯ vtctldclient --server "localhost:15999" AddCellsAlias -c zone1 -c zone2 megazone
Created cells alias: megazone (cells = [zone1 zone2])

Related Issue(s)

• Closes Add CellInfo and CellsAlias r/w rpcs to VtctldServer #8216

Checklist

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

Deployment Notes

[doeg]

Just curious: why is the --cells flag defined twice?

[ajm188]

I am still unsure how to best convey this in usage strings, but pflag.StringSlice lets you do:

-x "a,b"        # result: [a, b]
-x "a" -x "b"   # result: [a, b]
-x "a,b" -x "c" # result: [a, b, c]