adding spack diff command (#22283)

A `spack diff` will take two specs, and then use the spack.solver.asp.SpackSolverSetup to generate
lists of facts about each (e.g., nodes, variants, etc.) and then take a set difference between the
two to show the user the differences.

Example output:

    $ spack diff python@2.7.8 python@3.8.11
     ==> Warning: This interface is subject to change.

     --- python@2.7.8/tsxdi6gl4lihp25qrm4d6nys3nypufbf
     +++ python@3.8.11/yjtseru4nbpllbaxb46q7wfkyxbuvzxx
     @@ variant_value @@
     -  python patches a8c52415a8b03c0e5f28b5d52ae498f7a7e602007db2b9554df28cd5685839b8
     +  python patches 0d98e93189bc278fbc37a50ed7f183bd8aaf249a8e1670a465f0db6bb4f8cf87
     @@ version @@
     -  openssl Version(1.0.2u)
     +  openssl Version(1.1.1k)
     -  python Version(2.7.8)
     +  python Version(3.8.11)

Currently this uses diff-like output but we will attempt to improve on this in the future.

One use case for `spack diff` is whenever a user has a disambiguate situation and cannot 
remember how two different installs are different. The command can also output `--json` in
the case of a more analysis type use case where we want to save complete data with all
diffs and the intersection. However, the command is really more intended for a command
line use case, and we likely will have an analyzer more suited to saving data

Signed-off-by: vsoch <vsoch@users.noreply.github.com>

Co-authored-by: vsoch <vsoch@users.noreply.github.com>
Co-authored-by: Tamara Dahlgren <35777542+tldahlgren@users.noreply.github.com>
Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

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