Peer role should be string instead of number

[BusyJay]

Bug Report

What did you do?

Enable joint consensus and check some region in joint state

What did you expect to see?

Peers show with exact role name.

What did you see instead?

Peers with role set to number:

{
  "id": 5995753,
  "store_id": 8,
  "role": 2
}

What version of PD are you using (pd-server -V)?

Nightly

[HunDunDM]

Modification from kvproto has some impact and may introduce compatibility issues.

[HunDunDM]

Is it ok to only adjust the output of pd-ctl?

[BusyJay]

It may be another problem that output of pd-ctl is not consistent with curl.