Add DATASETTE_SSL_KEYFILE and DATASETTE_SSL_CERTFILE envvars

[asg017]

For the --ssl-keyfile and --ssl-certfile flags

Closes #2422

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📚 Documentation preview 📚: https://datasette--2423.org.readthedocs.build/en/2423/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.65%. Comparing base (0bc6a2a) to head (611d665).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2423      +/-   ##
==========================================
+ Coverage   92.60%   92.65%   +0.04%     
==========================================
  Files          41       41              
  Lines        6425     6425              
==========================================
+ Hits         5950     5953       +3     
+ Misses        475      472       -3     

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