UI for dataset-selection in cubeviz spectral extraction

[kecnry]

Description

This pull request adds the missing UI dropdown for input cube selection in cubeviz's spectral extraction.

Currently milestoning as a bug since the API existed, but we should test to make sure that there aren't unintended consequences since elsewhere we do assume only a single INPUT flux cube (but maybe this is safe because it is guaranteed to be the shape).

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)?

[camipacifici]

Tested a few things and could not break it. Works for me, thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.51%. Comparing base (329c969) to head (f13fe0b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3411   +/-   ##
=======================================
  Coverage   87.51%   87.51%           
=======================================
  Files         128      128           
  Lines       19901    19901           
=======================================
  Hits        17417    17417           
  Misses       2484     2484           

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