[Feature] Custom config handler, support callback when set mutable config at FE and BE

[ccoffline]

Is your feature request related to a problem? Please describe.
We need to custom query traffic limit rules in FE, but config can only be basic type and no callback. We need to compile the limit rules when set new limit rules config, and calculate all the rules at each query.

Describe the solution you'd like
Add a handler field at ConfField named handler to specify the class to use as callback function.
Call the handler when setting new config value. The default handler is the same as current.
Support all types config.