Add `spack --bootstrap` option for accessing bootstrap store (#25601)

We can see what is in the bootstrap store with `spack find -b`, and you can clean it with `spack
clean -b`, but we can't do much else with it, and if there are bootstrap issues they can be hard to
debug.

We already have `spack --mock`, which allows you to swap in the mock packages from the command
line. This PR introduces `spack -b` / `spack --bootstrap`, which runs all of spack with
`ensure_bootstrap_configuration()` set. This means that you can run `spack -b find`, `spack -b
install`, `spack -b spec`, etc. to see what *would* happen with bootstrap configuration, to remove
specific bootstrap packages, etc. This will hopefully make developers' lives easier as they deal
with bootstrap packages.

This PR also uses a `nullcontext` context manager. `nullcontext` has been implemented in several
other places in Spack, and this PR consolidates them to `llnl.util.lang`, with a note that we can
delete the function if we ever reqyire a new enough Python.

- [x] introduce `spack --bootstrap` option
- [x] consolidated all `nullcontext` usages to `llnl.util.lang`

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