Add SBDebugger::GetSetting() public APIs

This patch adds new SBDebugger::GetSetting() API which
enables client to access settings as SBStructedData.

Implementation wise, a new ToJSON() virtual function is added to OptionValue
class so that each concrete child class can override and provides its
own JSON representation. This patch aims to define the APIs and implement
a common set of OptionValue child classes, leaving the remaining for
future patches.

This patch is used later by auto deduce source map from source line breakpoint
feature for testing generated source map entries.

Differential Revision: https://reviews.llvm.org/D133038