Introduce gdb.ValuePrinter

There was an earlier thread about adding new methods to
pretty-printers:

https://sourceware.org/pipermail/gdb-patches/2023-June/200503.html

We've known about the need for printer extensibility for a while, but
have been hampered by backward-compatibilty concerns: gdb never
documented that printers might acquire new methods, and so existing
printers may have attribute name clashes.

To solve this problem, this patch adds a new pretty-printer tag class
that signals to gdb that the printer follows new extensibility rules.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30816


Reviewed-By: Eli Zaretskii <eliz@gnu.org>