parse duplicate query string fields for EventV2

[woodybriggs]

Description

AWS Gateway Proxy V2 Integrations with lambdas serializes duplicate fields in query string by concatenating the values together. This is not reflected in the current code.

Before this change:

http://example.com/endpoint?filter=1&filter=2
Would be serialized as 

{
    "filter": 2
}

After this change

{
    "filter": "1,2"
}

Motivation and Context

https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html#http-api-develop-integrations-lambda.proxy-format~

extract from aws

Format 2.0 doesn't have multiValueHeaders or multiValueQueryStringParameters fields. Duplicate headers are combined with commas and included in the headers field. Duplicate query strings are combined with commas and included in the queryStringParameters field.

fixes -> #1437

How Has This Been Tested?

no tests modified or created. happy to modify some tests if i can be pointed to the current test for this problem statement.

Screenshots (if appropriate):

[dnalborczyk]

thank you @woodybriggs could you add some tests?

[dnalborczyk]

@woodybriggs someone picked up your PR and added some tests to it: #1525

thanks again, closing this for good house keeping.