Prevent moving any followers or leaders to TiFlash nodes

[disksing]

Feature Request

Describe your feature request related problem:

TiFlash does not allow to have any followers or leaders. We use placement rules to prevent moving followers or leaders to TiFlash nodes. If user writes wrong config, or PD creates wrong schedule operator, it can still happen and causes TiFlash to crash.
Related to #2237

Describe the feature you'd like:

Support configure the ability of node. If a node cannot have any followers, PD can reject operators that want to violate configuration.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

Related features: label constraints, placement rules, label properties, specialUse filter.