service: Adds prototyped operations from APIGateWay and S3 service that uses rest encoder utility

[skotambkar]

• Adds prototype for getApiKey request for apigateway service and putObject request for s3 service that use rest encoder utility from PR aws/aws-sdk-go-v2#449
• This PR adds changes on top of changes added for PR aws/aws-sdk-go-v2#441, also takes in changes from PR aws/aws-sdk-go-v2#449.
• We add Marshal.go for service/apigateway & service/s3. It contains prototyped marshaler operations and functions for respective operations.
• We also add test to compare the marshaled request using rest encoder utility with the old rest marshaler.

[jasdel]

Once the types packages is merged in it will be easier to review this prototype :)

[jasdel]

For this PR what do you think about moving the apigateway/internal/awsrestjson package content into the API client's package, apigateway?. This would allow us to continue making progress on marshaling prototypes and merge this PR before we finalize on the client's package structure.