Extend the Properties api to better support nested configuration

[soulomoon]

          > I don't know whether we might want more configuration options later, but perhaps it would be wise to nest all these in config.tokenMapping or something?

I think this one would be better. I am going to extend the properties api to better support nested configuration.

Originally posted by @soulomoon in #3940 (comment)

Since the plugin api's support for nested configuration is poor. It would be better to extend it to enable a type safe configuration for nested configuration.

Steps

Towards this goal there are two steps of implementation.

1. The current implementation use (unsafecoerce), and the safty is provided by hand obeying the rules in property map.
   Which is not really safe. We replace it with safer implementation. doing in Properties API: Remove unsafe coerce in favor of type class based method in  #3947
2. Extend the configuration api under the above safer implementation to provide nicer nested configuration. doing in 3944 extend the properties api to better support nested configuration #3952

It should support #3937 better

[michaelpj]

FWIW I think this is probably not the most important thing to do. We have generally coped with one layer of custom plugin config, and if you need more you can fake it by adding prefixes/suffixes to option names. Not lovely, but not worth a huge amount of effort to fix, IMO.

[soulomoon]

No worries. Things seems to be going smoother than I imagin, It is very close to be done now.