[lldb] Add OperatingSystem base class to the lldb python module

This patch introduces an `OperatingSystem` base implementation in the
`lldb` python module to make it easier for lldb users to write their own
implementation.

The `OperatingSystem` base implementation is derived itself from the
`ScriptedThread` base implementation since they share some common grounds.

To achieve that, this patch makes changes to the `ScriptedThread`
initializer since it gets called by the `OperatingSystem` initializer.

I also took the opportunity to document the `OperatingSystem` base
class and methods.

Differential Revision: https://reviews.llvm.org/D159315



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>