[REQ][GO] AWSv4 signature

Created by: jerome-jutteau

Is your feature request related to a problem? Please describe.

In order to make calls to an API using Access Key and Secret Key signature, I would need to sign my requests using AWSv4 signature

Describe the solution you'd like

I propose to use official github.com/aws/aws-sdk-go/aws/signer/v4 and patch go templates in order to let users write something like:

	auth := context.WithValue(context.Background(), mygen.ContextAWSv4, mygen.AWSv4{
		AccessKey: os.Getenv("MY_ACCESS_KEY"),
		SecretKey: os.Getenv("MY_SECRET_KEY"),
	})