lambdaurl: Populate RemoteAddr with SourceIP

[lyoung-confluent]

Currently when using the lamdbaurl package the RemoteAddr field of the parsed *http.Request is not populated:

// RemoteAddr allows HTTP servers and other software to record
// the network address that sent the request, usually for
// logging. This field is not filled in by ReadRequest and
// has no defined format. The HTTP server in this package
// sets RemoteAddr to an "IP:port" address before invoking a
// handler.
// This field is ignored by the HTTP client.
RemoteAddr string

This PR changes the behavior to populate the value from the underlying request context. This matches the behavior of other AWS libraries: https://github.com/awslabs/aws-lambda-go-api-proxy/blob/09c9b90f1fd1a45c05eb3a2a7a166526fcddabd8/core/requestv2.go#L168

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cce0845) 72.93% compared to head (468b645) 72.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #546      +/-   ##
==========================================
+ Coverage   72.93%   72.95%   +0.01%     
==========================================
  Files          26       26              
  Lines        1456     1457       +1     
==========================================
+ Hits         1062     1063       +1     
  Misses        322      322              
  Partials       72       72              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[bmoffatt]

Nice, thanks!