support 'SET CONFIG' syntax to change configs of TiKV/PD instances

[qw4990]

Development Task

After implementing this proposal(#13660), TiKV and PD support to dynamic configs.
Now we can get PD and TiKV instances' configs from PD according to pd-ctl.
Using pd-ctl is not convenient for our user, so we need to support to show configs by SQL.

After discussing, we decide to support these syntaxes:

1. set config instance/type key=value

For example:

1. set config tikv log.level='info'
2. set config address log.level='info'

Our parser has already supported this syntax now.(pingcap/parser#768)

Teachability, Documentation, Adoption, Migration Strategy:

#13660