This patch adds a finalize method which destroys the underlying RAII
SBProgress. My primary motivation for this is so I can write better
tests that are non-flaky, but after discussing with @clayborg in my DAP
message improvement patch (#124648) this is probably an essential API
despite that I originally argued it wasn't.
I wanted to clarify the semantics around SBProgress. Given the nature of
Progress events, copying seems like the wrong idea. Making SBProgress
move-only (like SBStream) seems like the better choice here.
Recently I've been working on a lot of internal Python tooling, and in
certain cases I want to report async to the script over DAP. Progress.h
already handles this, so I've exposed Progress via the SB API so Python
scripts can also update progress objects.
I actually have no idea how to test this, so I just wrote a [toy command
to test
it](https://gist.github.com/Jlalond/48d85e75a91f7a137e3142e6a13d0947)
I also copied the first section of the extensive Progress.h class
documentation to the docstrings.
