Introduce a new privilege for the set config statement

[qw4990]

Feature Request

Describe the feature you'd like:

After #768, we can use set config on TiDB to update configs of TiKV/PD.
Now the privilege of this statement is PrivSuper, which is not appropriate.
We should introduce a new privilege for it.

Describe alternatives you've considered:

No

[kennytm]

if not super privilege, would that user be allowed to change privileged settings like:?

set config security.skip-grant-table = true;

[qw4990]

if not super privilege, would that user be allowed to change privileged settings like:?

set config security.skip-grant-table = true;

@tiancaiamao Could you please give us some suggestions about this case...

[qw4990]

if not super privilege, would that user be allowed to change privileged settings like:?

set config security.skip-grant-table = true;

After discussing with @tiancaiamao , we decide to handle security config items especially, which require both SuperPriv and SetConfPriv when modifying.

[tiancaiamao]

Allow the modification of skip-grant-table dynamically would become a vulnerability.
Skip grant table is a special config which is used for disaster recovering from user's misoperation, maybe we do not have the necessity to dynamic change this config.