Create import_region method for loading subsets

[javerbukh]

Description

This pull request adds an import_region method which will be able to load subsets in the following formats:

• [x] List
• [x] Dictionary
• [ ] String
• [x] SpectralRegion object (covered by #2941 )

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[bot]]

Codecov Report

Attention: Patch coverage is 80.55556% with 7 lines in your changes missing coverage. Please review.

Project coverage is 88.73%. Comparing base (73e54d0) to head (035ef71). Report is 1 commits behind head on main.

Files	Patch %	Lines
...igs/default/plugins/subset_plugin/subset_plugin.py	80.55%	7 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3082      +/-   ##
==========================================
- Coverage   88.76%   88.73%   -0.03%     
==========================================
  Files         111      111              
  Lines       17246    17281      +35     
==========================================
+ Hits        15308    15334      +26     
- Misses       1938     1947       +9     

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