Deps: Python, bundle zstandard package

This package allows Python scripts to handle compressed blend files (see
rB2ea66af742bc). This is for example needed by Blender Asset Tracer to
send files to a Flamenco render farm.

This change includes a new `WITH_PYTHON_INSTALL_ZSTANDARD` build-time
option, to control whether to actually install the package. For this the
already-existing approach for Requests was copied.

Reviewed By: LazyDodo, mont29, brecht

Differential Revision: https://developer.blender.org/D12777