feat(event-handler): allow for cors=None setting

[michaelbrewer]

Issue #, if available:

Description of changes:

At the route level we should allow for cors=None which means that cors are enabled for this route when configured globally

Global Setting	Route Setting	Includes cors headers?
app=ApiGatewayResolver()	@app.get("/cors", cors=True)	Yes
app=ApiGatewayResolver(cors=CORSConfig())	@app.get("/cors", cors=True)	Yes
app=ApiGatewayResolver()	@app.get("/no-cors", cors=False)	No
app=ApiGatewayResolver(cors=CORSConfig())	@app.get("/no-cors", cors=False)	No
app=ApiGatewayResolver()	@app.get("/no-cors")	No
app=ApiGatewayResolver(cors=CORSConfig())	@app.get("/cors")	Yes

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Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

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[codecov-commenter]

Codecov Report

Merging #421 (f4bd418) into develop (d044463) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #421   +/-   ##
========================================
  Coverage    99.89%   99.89%           
========================================
  Files          101      101           
  Lines         3990     3994    +4     
  Branches       197      198    +1     
========================================
+ Hits          3986     3990    +4     
  Misses           1        1           
  Partials         3        3           

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

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