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`

share/spack/spack-completion.bash

@@ -335,7 +335,7 @@ _spacktivate() {
 _spack() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -H --all-help --color -c --config -C --config-scope -d --debug --show-cores --timestamp --pdb -e --env -D --env-dir -E --no-env --use-env-repo -k --insecure -l --enable-locks -L --disable-locks -m --mock -p --profile --sorted-profile --lines -v --verbose --stacktrace -V --version --print-shell-vars"
+        SPACK_COMPREPLY="-h --help -H --all-help --color -c --config -C --config-scope -d --debug --show-cores --timestamp --pdb -e --env -D --env-dir -E --no-env --use-env-repo -k --insecure -l --enable-locks -L --disable-locks -m --mock -b --bootstrap -p --profile --sorted-profile --lines -v --verbose --stacktrace -V --version --print-shell-vars"
     else
         SPACK_COMPREPLY="activate add analyze arch audit blame bootstrap build-env buildcache cd checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop diff docs edit env extensions external fetch find gc gpg graph help info install license list load location log-parse maintainers mark mirror module monitor patch pkg providers pydoc python reindex remove rm repo resource restage solve spec stage style tags test test-env tutorial undevelop uninstall unit-test unload url verify versions view"
     fi