Query parameter values not URL decoded when using HTTP API V2 Gateway with a RequestStreamHandler implementation

[jpfennelly]

Issue #, if available: 976

Description of changes:

• Fixed AwsProxyRequestBuilder, used by unit tests, to be immutable when calling the alb() method. Previously, a call to the alb() method would encode query parameters, modifying the state of the builder. Subsequent usage of the builder for non-ALB unit tests would use the now-encoded query parameters. This wasn't an issue in current current tests because no tests were testing query param decoding. Added unit tests for the builder class itself.
• Additional unit tests and cleanup to separate form param from query param constants in tests
• Added overloaded generateParameterMap() method that takes a flag to determine if it should decode query parameters so that functionality for ALB and non-ALB requests can be supported
• Updated unit tests to test encoded query parameter names and values
• Update HTTP request methods for parameters to handle decoding for ALB requests
• Updated HTTP API V2 request to correctly decode values

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.