deprecate conf-specific helper methods in specviz, specviz2d, cubeviz

[kecnry]

Description

This pull request deprecates config-specific helper methods in specviz, specviz2d, and cubeviz in preparation for deconfigging efforts.

Note that this intentionally defers deprecating:

• load_trace (requires load_data support)
• .specviz access (requires general parser logic)

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Attention: Patch coverage is 58.91473% with 53 lines in your changes missing coverage. Please review.

Project coverage is 87.53%. Comparing base (2084d96) to head (bf6e860).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...igs/default/plugins/model_fitting/model_fitting.py	61.62%	33 Missing ⚠️
jdaviz/core/helpers.py	25.00%	12 Missing ⚠️
jdaviz/configs/default/plugins/viewers.py	33.33%	4 Missing ⚠️
jdaviz/configs/specviz/helper.py	77.77%	2 Missing ⚠️
jdaviz/configs/cubeviz/helper.py	75.00%	1 Missing ⚠️
jdaviz/configs/imviz/helper.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3388      +/-   ##
==========================================
- Coverage   88.01%   87.53%   -0.49%     
==========================================
  Files         127      128       +1     
  Lines       19747    19884     +137     
==========================================
+ Hits        17381    17406      +25     
- Misses       2366     2478     +112     

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[pllim]

No alternative?

[kecnry]

ah, I had meant to either move this logic or postpone it. I'll revisit if its feasible to move to the plugin here or better as a follow-up effort (in which case I'll remove the deprecation for now).

I'll move back to draft for now.

[kecnry]

I think ultimately this needs to be absorbed into the (future) general load_data, so I'll make a note on that ticket and hold off on deprecating until that's in place.

[pllim]

Just a few nitpicks on wordings. If no more new deprecation you want to add, maybe it is ready to be taken back out of draft after a rebase to pick up CI fix.

[pllim]

LGTM. Thanks!