Add custom Decodable conformance

[twistinside]

Add custom Decodable conformance. Closes #73

Motivation:

Absent collections would fail to decode, when in reality it is possible to have empty headers, for example. This allows processing to continue in the context of the Lambda runtime where users can handle the collections as dictated by their code.

Modifications:

Custom Decodable conformance has been added to both API Gateway request objects to optionally decode absent collections to empty collections.

Unit testa have been added for the same.

Result:

This will make the swift lambda runtime more accepting of input, allowing processing to proceed in the rare cases where collection fields would be absent in the JSON received by the runtime. This will unlock certain functionality in edge cases such as testing lambda code from API Gateway console with empty headers (the use case that prompted this change).

This change will require users to update their code to remove optional handling for the collections involved, potentially also requiring extra logic to validate that the collections they've received aren't empty.