[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

Attention: Patch coverage is 53.69863% with 169 lines in your changes missing coverage. Please review.

Project coverage is 76.31%. Comparing base (5f76fda) to head (9c25147).

Files with missing lines	Patch %	Lines
jupyter_server/serverapp.py	43.79%	68 Missing and 9 partials ⚠️
...r/tests/unix_sockets/test_serverapp_integration.py	22.72%	68 Missing ⚠️
jupyter_server/utils.py	66.00%	15 Missing and 2 partials ⚠️
jupyter_server/tests/conftest.py	76.92%	2 Missing and 1 partial ⚠️
jupyter_server/base/handlers.py	77.77%	1 Missing and 1 partial ⚠️
jupyter_server/tests/unix_sockets/conftest.py	94.44%	0 Missing and 1 partial ⚠️
jupyter_server/tests/unix_sockets/test_api.py	97.29%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[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?