[lldb/Plugins] Add support for ScriptedThread in ScriptedProcess

This patch introduces the `ScriptedThread` class with its python
interface.

When used with `ScriptedProcess`, `ScriptedThreaad` can provide various
information such as the thread state, stop reason or even its register
context.

This can be used to reconstruct the program stack frames using lldb's unwinder.

rdar://74503836

Differential Revision: https://reviews.llvm.org/D107585



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>