feat!: Make route handlers functional decorators

[provinzkraut]

Make route handler decorators functional decorators.

This gets rids of the __call__ method having to mutate internal state and lets us clean up things a bit.

It also makes for a nicer syntax and improved type/runtime checking of the handler classes, because they can now receive a fn as a required keyword argument, which gets rid of the HandlerClass("/")(handler_fn) pattern.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.99%. Comparing base (8aae5d8) to head (dbe473f).

Additional details and impacted files

@@           Coverage Diff           @@
##             v3.0    #3436   +/-   ##
=======================================
  Coverage   97.98%   97.99%           
=======================================
  Files         326      326           
  Lines       14566    14559    -7     
  Branches     2314     2306    -8     
=======================================
- Hits        14273    14267    -6     
  Misses        154      154           
+ Partials      139      138    -1     

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