[lldb/Plugins] Add ScriptedProcess::GetThreadsInfo interface

This patch adds a new method to the Scripted Process interface to
retrive a dictionary of Scripted Threads. It uses the thread ID as a key
and the Scripted Thread instance as the value.

This dictionary will be used to create Scripted Threads in lldb and
perform calls to the python scripted thread object.

rdar://87427126

Differential Revision: https://reviews.llvm.org/D117068



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