[Proposal] Merge `ALL_PERMITTED` and `DATABASE_PERMITTED` into one privilege provider.

Hi community,

Now, in ShardingSphere-Proxy, we provide two privilege providers: `ALL_PERMITTED` and `DATABASE_PERMITTED`.
Some users are confused when using it, especially in cluster mode, modifying the local yaml file does not really change the authority configuration in cluster, which increases the difficulty of operation and maintenance.

Ref: https://shardingsphere.apache.org/document/current/en/user-manual/shardingsphere-proxy/yaml-config/authority/

Maybe we can combine these two providers:
1. Merge providers into one
2. Super user can be specified (the effect is the same as `ALL_PERMITTED`)
3. Non-super users must be authorized, otherwise they cannot operate any database.
4. The super user can execute `ALTER AUTHORITY RULE` through DistSQL to dynamically modify the configuration.

This is just a preliminary idea, more suggestions and discussions are welcome.