SL-20473 Add GenericStreamingMessage and dummy handler to suppress packet loss and log spam noise when visiting GLTF enabled regions.

DRTVWR-583

# Conflicts:
#	scripts/content_tools/anim_tool.py

SL-10163 - allow joint aliases in animation uploads. Names are canonicalized before sending to simulator.

# Conflicts:
#	indra/newview/llviewerassetupload.cpp
#	indra/newview/llvoavatar.cpp
#	scripts/content_tools/anim_tool.py

- When installing a 3P via use_prebuilt_binary it will pass --installdir=${AUTOBUILD_INSTALL_DIR} to
autobuild. On the other hand packages-formater was not passing this option, this could lead to subtle
errors when AUTOBUILD_INSTALL_DIR is different to what autobuild.xml stipulates.

This changeset makes it possible to build the Second Life viewer using
Python 3. It is designed to be used with an equivalent Autobuild branch
so that a developer can compile without needing Python 2 on their
machine.

Breaking change: Python 2 support ending

Rather than supporting two versions of Python, including one that was
discontinued at the beginning of the year, this branch focuses on
pouring future effort into Python 3 only. As a result, scripts do not
need to be backwards compatible. This means that build environments,
be they on personal computers and on build agents, need to have a
compatible interpreter.

Notes

- SLVersionChecker will still use Python 2 on macOS
- Fixed the message template url used by template_verifier.py

SL-16124 First pass at a Python3 script to launch multiple non-interactive Viewers with differing command line options