fix: Support multiple of same query string for LambdaProxyIntegrationEventV2

[drace-rgare]

Description

Instead of using multiValueQueryStringParameters API Gateway HTTP API combines duplicate query string keys with commas in the queryStringParameters field.

Motivation and Context

Issue #1437
Similar change to PR #1438 but woodybriggs never added the tests for it, and wasn't returning null for empty search params, one of the test scenarios.

Before this change:

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

{
    "filter": 2
}

After this change

{
    "filter": "1,2"
}

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

How Has This Been Tested?

New unit tests added, based off of scenarios from parseMultiValueQueryStringParameters tests

Screenshots (if appropriate):

[dnalborczyk]

thank you @drace-rgare !!