[Notebook port 4835] Add UNIX socket support to notebook server

[jtpio]

This is a port of jupyter/notebook#4835. (The changes could not be easily cherry-picked because it requires substantial changes when porting the tests.)

Overview of the changes:

• enables connection to a jupyter server via UNIX sockets.
• adds integration tests for launching, listing, and shutting-down UNIX socket servers.
• runs the integration tests in a new, separate Github workflow.
• adds a --integration-tests flag to the pytest CLI options for running the integration tests. Otherwise, these tests are ignored. (They shouldn't really be tested locally).
• improves, adds, and tests the handling of some public properties related to the ServerApp's URLs.
• adds some utility functions for making sync/async HTTP or UNIX socket requests to a running Jupyter server.

[codecov-commenter]

Codecov Report

Merging #525 (9c25147) into master (5f76fda) will decrease coverage by 0.66%.
The diff coverage is 53.69%.

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
- Coverage   76.97%   76.31%   -0.67%     
==========================================
  Files         107      110       +3     
  Lines        9496     9809     +313     
  Branches     1031     1067      +36     
==========================================
+ Hits         7310     7486     +176     
- Misses       1814     1943     +129     
- Partials      372      380       +8     

Impacted Files	Coverage Δ
...r/tests/unix_sockets/test_serverapp_integration.py	22.72% <22.72%> (ø)
jupyter_server/serverapp.py	64.88% <43.79%> (-1.89%)	⬇️
jupyter_server/utils.py	63.15% <66.00%> (+1.02%)	⬆️
jupyter_server/tests/conftest.py	78.57% <76.92%> (-21.43%)	⬇️
jupyter_server/base/handlers.py	63.91% <77.77%> (+0.22%)	⬆️
jupyter_server/tests/unix_sockets/conftest.py	94.44% <94.44%> (ø)
jupyter_server/tests/unix_sockets/test_api.py	97.29% <97.29%> (ø)
jupyter_server/__init__.py	64.28% <100.00%> (+2.74%)	⬆️
jupyter_server/tests/test_serverapp.py	99.09% <100.00%> (+0.11%)	⬆️
... and 5 more

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[kevin-bates]

I went ahead and dismissed the two remaining codeQL issues since they were only in test code (side effect stuff).

[Zsailer]

Thanks, @jtpio! 🚀 This looks good to me.

@kevin-bates or @mwakaba2, would either of you be willing to do a quick scan of the code to make sure nothing looks out of the ordinary? After one of you approves, I'll go ahead a merge and release. Thank you!

[kevin-bates]

These changes look good. Not sure if the dismissed codeQL issues in the test code will "come back to life" or not, but let's see. 😄

[lazka]

Is this actually used somewhere?