This expands on #128 to add another bootstrap tool subcommand: bootstrap-version. This is designed to just be a simple number that's bumped whenever there's a feature scripts might need to query for. The version number is also automatically checked against the (numeric) value of the PANTS_BOOTSTRAP_TOOLS key.

For instance, suppose the bootstrap tools already existed (at version 1), and then bootstrap-cache-key command was added, the version number would be bumped to 2, and then a consumer (such as pantsbuild/actions#6) could run something like:

PANTS_BOOTSTRAP_CACHE_KEY=$(PANTS_BOOTSTRAP_TOOLS=2 ./pants bootstrap-cache-key)

Notes:

• I don't personally have concrete uses cases for this, but potentially adding different output like Add 'bootstrap tools' (bootstrap-cache-key) via PANTS_BOOTSTRAP_TOOLS=1 #128 (review) might use it.
• The subcommand name is bootstrap-version rather than version to avoid conflict with the subcommand on real-pants (i.e. ./pants version without PANTS_BOOTSTRAP_TOOLS=1 works, and prints the underlying pants version like 2.13.0, whereas ./pants bootstrap-version errors out with Unknown goal: bootstrap-version)
• It's unfortunate I didn't add this in Add 'bootstrap tools' (bootstrap-cache-key) via PANTS_BOOTSTRAP_TOOLS=1 #128, because there may be people who installed the script supporting PANTS_BOOTSTRAP_TOOLS but not supporting this versioning. Maybe the window is small enough that that doesn't matter?