feat(apigateway): introduce specialized routers for typing

[rubenfonseca]

Issue number: #1781

Summary

Changes

Please provide a summary of what's being changed

This PR implements the following changes:

• Introduces specialized Router classes
• Reorganizes classes into a new router.py file
• Adds tests for new code
• Updates documentation

User experience

Please share what the user experience looks like before and after this change

Before the change, you would not get type hints from your IDE when accessing the current_event property of your splitted router.

After this change, you can use a specialized router for each kind of event source, and get access to the specialized events:

Checklist

If your change doesn't seem to apply, please leave them unchecked.

• Meet tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

Acknowledgment

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[codecov-commenter]

Codecov Report

Base: 97.59% // Head: 97.59% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (482263d) compared to base (788f39b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1824   +/-   ##
========================================
  Coverage    97.59%   97.59%           
========================================
  Files          141      142    +1     
  Lines         6434     6444   +10     
  Branches       444      444           
========================================
+ Hits          6279     6289   +10     
  Misses         123      123           
  Partials        32       32           

Impacted Files	Coverage Δ
aws_lambda_powertools/event_handler/router.py	100.00% <100.00%> (ø)

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[leandrodamascena]

Hello @rubenfonseca! Another incredible PR! Everything worked as expected here and I just added more details in the example just to make sure the user has a full example of use.