[lldb/Python] Make use of PythonObject and PythonFormat in callbacks (NFC)

This patch extends the template specialization of PythonFormat structs
and makes use of the pre-existing PythonObject class to format arguments
and pass them to the right method, before calling it.

This is a preparatory patch to merge PythonFormat with SWIGPythonBridge's
GetPythonValueFormatString methods.

Differential Revision: https://reviews.llvm.org/D138248



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