[lldb/bindings] Add Python ScriptedProcess base class to lldb module

In order to facilitate the writting of Scripted Processes, this patch
introduces a `ScriptedProcess` python base class in the lldb module.

The base class holds the python interface with all the - abstract -
methods that need to be implemented by the inherited class but also some
methods that can be overwritten.

This patch also provides an example of a Scripted Process with the
`MyScriptedProcess` class.

rdar://65508855

Differential Revision: https://reviews.llvm.org/D95712



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