solara standalone: default to host='localhost' and allow mult instances

[kecnry]

Description

This pull request defaults to using host='localhost' when launching solara, but allowing the user to override from the CLI, and also uses an open port by default, allowing for multiple instances.

The port=0 default requires a release and bump in pin to solara.... marking as draft until then.

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)?

[pllim]

Code looks reasonable, so LGTM as long as this works for @camipacifici . Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.40%. Comparing base (8b3f3a1) to head (9646571).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
jdaviz/cli.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3176      +/-   ##
==========================================
- Coverage   88.41%   88.40%   -0.01%     
==========================================
  Files         124      124              
  Lines       18347    18349       +2     
==========================================
  Hits        16221    16221              
- Misses       2126     2128       +2     

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[kecnry]

The port=0 default requires a release and bump in pin to solara.... marking as draft until then.