Add options lambdaurl.WithDetectContentType and lambda.WithContextValue

[bmoffatt]

Issue #, if available:

#508

Description of changes:

Adds an option to utilize the stblib http.DetectContentType on the first Write to a lambda url response writer. This is done by adding a lambda.WithContextValue as a mechinisim to squirrel arbitrary context values in StartWithOptions wrappers like lambdaurl.Start

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[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (1dca084) 72.90% compared to head (0ea7933) 72.93%.

Files	Patch %	Lines
lambda/handler.go	12.50%	6 Missing and 1 partial ⚠️
lambdaurl/http_handler.go	94.59%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #516      +/-   ##
==========================================
+ Coverage   72.90%   72.93%   +0.03%     
==========================================
  Files          26       26              
  Lines        1421     1456      +35     
==========================================
+ Hits         1036     1062      +26     
- Misses        314      322       +8     
- Partials       71       72       +1     

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