Add version bootstrap tool subcommand, for capability checking

[huonw]

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 https://github.com/pantsbuild/actions/pull/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 https://github.com/pantsbuild/setup/pull/128#pullrequestreview-1131761160 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 #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?

[benjyw]

• 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?

I think it's fine - window is small.

• The checking could be packaged into the bootstrap script itself: the bootstrap script check that SCRIPT_VERSION >= PANTS_BOOTSTRAP_TOOLS, i.e. the example scenario above could be collapsed to just: PANTS_BOOTSTRAP_CACHE_KEY=$(PANTS_BOOTSTRAP_TOOLS=2 ./pants bootstrap-cache-key). Thoughts?

That could be neat! So the script runs if it's later than PANTS_BOOTSTRAP_TOOLS and fails otherwise?

[huonw]

That could be neat! So the script runs if it's later than PANTS_BOOTSTRAP_TOOLS and fails otherwise?

Yeah, exactly. I've implemented something along those lines.

[huonw]

Oops, thanks for that.