feat: provide access to arbitrary interpreters (#2507)

There are some use cases that folks want to cover here. They are
discussed in [this Slack thread][1]. The high-level summary is:
1. Users want to run the exact same interpreter that Bazel is running
   to minimize environmental issues.
2. It is useful to pass a target label to third-party tools like mypy
   so that they can use the correct interpreter.

This patch adds to @rickeylev's work from #2359 by adding docs
and a few integration tests.

[1]: https://bazelbuild.slack.com/archives/CA306CEV6/p1730095371089259

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Co-authored-by: Richard Levasseur <rlevasseur@google.com>