aws/signer/v4: Move AWS V4 Signer into public package (#735)

* aws/signer/v4: Move AWS V4 Signer into public package

Moves the private/signer/v4 into the aws/signer/v4 package, exposing it
for general use.

* aws/signer/v4: Update signers documentation and request handler name

Updates the documentation for the V4 signer, along with renaming the
request handler's name to reduce confusion between it and signing
request outside of the context of the SDK.

* private/model/api: Update code gen to use public v4 signer

* private/signer/v4: Remove old private v4 signer

private/signer/v4/functional_test.go → aws/signer/v4/functional_test.go

@@ -6,11 +6,10 @@ import (
 	"testing"
 	"time"
 
-	"github.com/stretchr/testify/assert"
-
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/awstesting/unit"
 	"github.com/aws/aws-sdk-go/service/s3"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestPresignHandler(t *testing.T) {