Enable specifying SendDecorators on a client

[jhendrixMSFT]

Added field SendDecorators to autorest.Client; this value will be
preferred over the default SendDecorators.
Added method Client.Send() which includes logic for using custom
SendDecorator chains.

Thank you for your contribution to Go-AutoRest! We will triage and review it as soon as we can.

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[catalinaperalta]

lgtm

[jhendrixMSFT]

@feiskyer PTAL, will wait for your approval before merging.

[jhendrixMSFT]

Accompanying code generator change: Azure/autorest.go#261

[feiskyer]

This looks great.

@jhendrixMSFT could you share an example how could it be used for Azure Go SDK clients?

[jhendrixMSFT]

@feiskyer here's how you would set a custom retry policy on a client. There are similar examples in the test added in this PR.

func MyCustomRetryPolicy(attempts int, backoff time.Duration, codes ...int) autorest.SendDecorator {
    return func(s autorest.Sender) autorest.Sender {
        return autorest.SenderFunc(func(r *http.Request) (*http.Response, error) {
            // do work before sending the request to the next SendDecorator here ...
 
            // pass the request to the next SendDecorator
            resp, err := s.Do(r)
 
            // do something with the response
            if resp.StatusCode == http.StatusTooManyRequests {
                // being throttled, handle as required....
            }
 
            // return response/error to my caller
            return resp, err
        })
    }
}
 
func main() {
    a, err := auth.NewAuthorizerFromEnvironment()
    if err != nil {
        panic(err)
    }
    client := redis.NewClient(subscriptionID)
    client.SendDecorators = []autorest.SendDecorator{MyCustomRetryPolicy(3, 5*time.Second, http.StatusInternalServerError)}
    client.Authorizer = a
 
    // sync example
    result, err := client.CheckNameAvailability(context.Background(), redis.CheckNameAvailabilityParameters{})
    if err != nil && result.IsHTTPStatus(http.StatusTooManyRequests) {
        // being throttled ...
    }
}

[feiskyer]

@jhendrixMSFT cool, thanks for the excellent example