replaces #9581

The difference is that this PR adds our fork of keepkeylib as a git submodule, instead of including it directly into the git tree. This avoids polluting the tree.

Our fork of keepkeylib has the protobuf-generated code regenerated with more modern protoc, and a lot of altcoin stuff is removed.

Also see description of prev PR:

This adds keepkeylib into our git tree. We will vendor it that way from now, instead of declaring it as an external dependency.

Upstream keepkeylib is effectively unmaintained -- but the only problem with it is that it requires an old version of protobuf. (see keepkey/python-keepkey#146)
By vendoring it, we can fix that ourselves.

keepkeylib is taken from PyPI, at keepkey==6.3.1, which is the version we have sha256 pinned for 5 years.

This also vendors the corresponding source protobuf files, which were not part of the keepkeylib sdist. Those are taken from the git repo, at the corresponding commit (for v6.3.1).

Subsequent commits

• strip out some of the shitcoin crap from keepkeylib, though only the parts that were easy to remove
• regenerate the pb2 files using protoc-21.12 (which is the version currently in debian stable, already much newer than what was previously used)

closes #7922