support 'SHOW CONFIG' syntax to show configs of PD and TiKV instances

[qw4990]

Feature Request

Describe the feature you'd like:

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. show config [where ...]

For example:

1. show config
2. show config where type="tikv"

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

Teachability, Documentation, Adoption, Migration Strategy:

#13660