Add liveness endpoint

[philippjfr]

Adds an optional endpoint to the Panel server that allows checking whether the server or a particular endpoint of the server is live.

[codecov]

Codecov Report

Merging #3832 (e3db6b7) into master (e43b8c5) will decrease coverage by 48.82%.
The diff coverage is 24.32%.

❗ Current head e3db6b7 differs from pull request most recent head 9c9db94. Consider uploading reports for the commit 9c9db94 to get more accurate results

@@             Coverage Diff             @@
##           master    #3832       +/-   ##
===========================================
- Coverage   82.98%   34.16%   -48.83%     
===========================================
  Files         221      222        +1     
  Lines       32339    32354       +15     
===========================================
- Hits        26838    11053    -15785     
- Misses       5501    21301    +15800     

Flag	Coverage Δ
ui-tests	34.16% <24.32%> (-0.02%)	⬇️
unitexamples-tests	?

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Impacted Files	Coverage Δ
panel/io/document.py	27.64% <12.50%> (-38.45%)	⬇️
panel/io/liveness.py	26.08% <26.08%> (ø)
panel/command/serve.py	16.08% <33.33%> (-19.26%)	⬇️
panel/__main__.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/katex.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/mathjax.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/perspective.py	0.00% <0.00%> (-100.00%)	⬇️
panel/pane/vtk/synchronizable_deserializer.py	0.00% <0.00%> (-97.59%)	⬇️
panel/models/ace.py	0.00% <0.00%> (-91.67%)	⬇️
panel/tests/widgets/test_input.py	8.86% <0.00%> (-91.14%)	⬇️
... and 156 more

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