allow overwriting existing registry entries

[kecnry]

Description

This pull request adds support for overwriting existing registry entries, which is particularly useful for downstream apps that want to modify something already in the registry without having to give it a new name.

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 91.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.92%. Comparing base (af8d1b8) to head (f9c348e).
Report is 4 commits behind head on main.

Files	Patch %	Lines
jdaviz/core/registries.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2744      +/-   ##
==========================================
+ Coverage   88.85%   88.92%   +0.07%     
==========================================
  Files         108      108              
  Lines       15920    16003      +83     
==========================================
+ Hits        14145    14230      +85     
+ Misses       1775     1773       -2     

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[pllim]

Maybe add one more sentence to explain what would happen to the old thing that was overwritten?

[kecnry]

suggestions for wording? It is replacing it in the registry, so calls to access the item will retrieve the new one instead.

[pllim]

Is the old one going to be destroyed somewhere or just sitting in purgatory? I am not familiar with this machinery.

[kecnry]

This doesn't do anything to destroy it, no, it just replaces the reference in the registry.

[pllim]

Should we destroy it?

[kecnry]

I don't see any reason to, its generally just a class and shouldn't be instantiated yet

[pllim]

Code looks like code. Thanks!

[pllim]

It is Friday and the weather is nice, let's get this in. 😆