feat: Add ability to customize authorization method

[andyland]

Before submitting

• [No] Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Currently, the only method to secure a LitServe endpoint is by providing an X-API-Key, which must be a global value. This excludes a wide-range of common authorization methods. This PR adds the ability to provide your own authorization method to a LitAPI. Example usage is added to the docs and tests

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (0c91d64) to head (876729b).

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #151   +/-   ##
===================================
  Coverage    79%    79%           
===================================
  Files        13     13           
  Lines       931    933    +2     
===================================
+ Hits        733    737    +4     
+ Misses      198    196    -2     

[lantiga]

Looks perfect @andyland! Stamping immediately