Add `ls_mean` in `gp` as user settable object

[athowes]

Follows from discussion in CDCgov/cfa-epinow2-pipeline#261.

We discussed that we could either do

1. As constant in here:

   cfa-config-generator/src/cfa_config_generator/utils/epinow2/constants.py

   Line 1 in 99cfb34

   shared_params = {

2. As user settable in https://github.com/CDCgov/cfa-config-generator/blob/99cfb3472c1517ba0551d666e87e84661678b45b/pipelines/epinow2/generate_config.py

Looks like we prefer 2.

[zsusswein]

Many of the past design decisions here were driven by the limitations of the GitHub Actions API. By moving this to a local library, we've freed ourselves of those constraints and can make some different choices.

Some notes from past discussions on what we might do:

• @kgostic had thoughts about a local file with our parameters checked into version control. We could edit this file.
• I worry about this file being hard to manage, but I like something in this vein. Maybe running the config generator writes a human-readable file we can edit and then it cleans the file after?
• I believe STF does something like this and we should check in with them how they do it
• Or we have a file the specifies all deviations from defaults?
• I think we should have the config generator be chattier and output more nice CLI messages using @natemcintosh's approach
• We could also have arguments for everything and some defaults. This is probably simplest

[athowes]

This is great stuff @zsusswein. It's possible we might want to move this to a broader conversation and keep focused on the GP lengthscale issue here. It's also possible that we should first update our general framework, then deal with issues like this GP one. Deferring here to e.g. @kgostic.