Add support for `smithy-rpc-v2-cbor` protocol

[jderusse]

Some services use this protocol (CloudWatch) and this is required to generate code

See https://github.com/aws/aws-sdk-php/blob/6ce549c3efe117e76f1f878ae8463d95b838ef9d/src/data/monitoring/2010-08-01/api-2.json

See PR for protocol implementation in AWS SDK aws/aws-sdk-php#3223

[stobrien89]

This will probably be more helpful: https://smithy.io/2.0/additional-specs/protocols/smithy-rpc-v2.html

I took some liberties in our implementation for performance reasons, so do whatever makes sense for you.

[stof]

The release notes for Cloudwatch when adding new protocols are explicitly saying that SDKs will choose between json and smithy-rpc-v2-cbor depending on what is most efficient for them: https://github.com/aws/aws-sdk-php/releases/tag/3.367.0

Given that json_decode is native in PHP while decoding cbor is not, I'm not sure this is needed yet. The official PHP SDK of AWS does not support smithy-rpc-v2-cbor either in its SupportedProtocols enum.
And most SDKs are actually using the AWS-JSON protocol for Cloudwatch: https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch-protocols-faq.html#getting-started