argon2 as an optional dependency

[vidartf]

This is a follow up to #450.

In setups where passwords are not used, it is preferential to have argon2 be an optional dependency:

• If not used, let us not import it (less resources)
• argon2 has C-extensions, so it should ideally not be a hard requirement unless we can avoid it
• This is also closer to how it was initially implemented in the notebook repository

It should still be a install requirement, as it is used by default.

[codecov-commenter]

Codecov Report

Merging #532 (0a42f89) into master (19ab0a9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
+ Coverage   76.31%   76.33%   +0.02%     
==========================================
  Files         110      110              
  Lines        9809     9809              
  Branches     1067     1067              
==========================================
+ Hits         7486     7488       +2     
+ Misses       1943     1942       -1     
+ Partials      380      379       -1     

Impacted Files	Coverage Δ
jupyter_server/auth/security.py	76.00% <100.00%> (ø)
jupyter_server/tests/services/kernels/test_api.py	99.01% <0.00%> (+1.96%)	⬆️

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[Zsailer]

Thanks, @vidartf!