gdb: add command 'tt-mp' to print MsgPack (with tt extensions)

Supported extensions:
- decimal
- uuid
- datetime
- error
- compression (only shows compression type and raw/compressed data size)
- interval

This functionality is similar to 'mp_str' function, but 'mp_str' can be
invoked from gdb only when a process is alive, so to be able to print
MsgPack during post-mortem analysis of core dump, 'mp_str' and all
subsequent functions that decode and convert MsgPack into human-readable
string were adapted to Python and put into the extension.

In order to simplify implementation and maintenance of the gdb-extension
implementation of the most of functions is kept as close to possible to
their 'C'-counterparts (original 'C'-name can be found as a comment at
the end of 'def' string)

Part of #7729

NO_DOC=gdb extension
NO_CHANGELOG=gdb extension
NO_TEST=gdb extension