Support set config ... and show config ... syntaxes

[qw4990]

Feature Request

Is your feature request related to a problem? Please describe:

No.

Describe the feature you'd like:

After implementing this proposal(pingcap/tidb#13660), TiDB, TiKV, and PD all support to change configs dynamically online.
Now we can update configs dynamically according to pd-ctl, which is not convenient to our user, so we need to support updating configs by SQL.
After discussing, we decide to support these syntaxes:

1. set config {component or instance} key=val
2. show config where ...

For example:

1. set config TiDB Log.Level="info"
2. set config '127.0.0.1:3306' Log.Level="info"
3. show config where type="TiDB"

[qw4990]

Close due to finish.