Include concrete environments with `include_concrete` (#33768)

Add the ability to include any number of (potentially nested) concrete environments, e.g.:

```yaml
   spack:
     specs: []
     concretizer:
         unify: true
     include_concrete:
     - /path/to/environment1
     - /path/to/environment2
```

or, from the CLI:

```console
   $ spack env create myenv
   $ spack -e myenv add python
   $ spack -e myenv concretize
   $ spack env create --include-concrete myenv included_env
```

The contents of included concrete environments' spack.lock files are
included in the environment's lock file at creation time. Any changes
to included concrete environments are only reflected after the environment
is re-concretized from the re-concretized included environments.

- [x] Concretize included envs
- [x] Save concrete specs in memory by hash
- [x] Add included envs to combined env's lock file
- [x] Add test
- [x] Update documentation

    Co-authored-by: Kayla Butler <<butler59@llnl.gov>
    Co-authored-by: Tamara Dahlgren <35777542+tldahlgren@users.noreply.github.co
m>
    Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>
    Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

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