service/transfer: Refactor tagging logic to keyvaluetags package

Reference: #10688

Output from acceptance testing:

```
--- PASS: TestAccAWSTransferUser_UserName_Validation (5.86s)
--- PASS: TestAccAWSTransferServer_disappears (11.20s)
--- PASS: TestAccAWSTransferUser_disappears (14.61s)
--- PASS: TestAccAWSTransferServer_forcedestroy (15.39s)
--- PASS: TestAccAWSTransferUser_basic (16.12s)
--- PASS: TestAccAWSTransferServer_apigateway (20.54s)
--- PASS: TestAccAWSTransferServer_basic (21.48s)
--- PASS: TestAccAWSTransferUser_modifyWithOptions (36.18s)
--- PASS: TestAccAWSTransferServer_vpcEndpointId (77.08s)
```

aws/resource_aws_transfer_server.go

@@ -8,10 +8,10 @@ import (
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/transfer"
-
 	"github.com/hashicorp/terraform-plugin-sdk/helper/resource"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/schema"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/validation"
+	"github.com/terraform-providers/terraform-provider-aws/aws/internal/keyvaluetags"
 )
 
 func resourceAwsTransferServer() *schema.Resource {
@@ -110,7 +110,7 @@ func resourceAwsTransferServer() *schema.Resource {
 
 func resourceAwsTransferServerCreate(d *schema.ResourceData, meta interface{}) error {
 	conn := meta.(*AWSClient).transferconn
-	tags := tagsFromMapTransfer(d.Get("tags").(map[string]interface{}))
+	tags := keyvaluetags.New(d.Get("tags").(map[string]interface{})).IgnoreAws().TransferTags()
 	createOpts := &transfer.CreateServerInput{}
 
 	if len(tags) != 0 {
@@ -192,7 +192,7 @@ func resourceAwsTransferServerRead(d *schema.ResourceData, meta interface{}) err
 	d.Set("identity_provider_type", resp.Server.IdentityProviderType)
 	d.Set("logging_role", resp.Server.LoggingRole)
 
-	if err := d.Set("tags", tagsToMapTransfer(resp.Server.Tags)); err != nil {
+	if err := d.Set("tags", keyvaluetags.TransferKeyValueTags(resp.Server.Tags).IgnoreAws().Map()); err != nil {
 		return fmt.Errorf("Error setting tags: %s", err)
 	}
 	return nil
@@ -249,8 +249,11 @@ func resourceAwsTransferServerUpdate(d *schema.ResourceData, meta interface{}) e
 		}
 	}
 
-	if err := setTagsTransfer(conn, d); err != nil {
-		return fmt.Errorf("Error update tags: %s", err)
+	if d.HasChange("tags") {
+		o, n := d.GetChange("tags")
+		if err := keyvaluetags.TransferUpdateTags(conn, d.Get("arn").(string), o, n); err != nil {
+			return fmt.Errorf("error updating tags: %s", err)
+		}
 	}
 
 	return resourceAwsTransferServerRead(d, meta)

aws/resource_aws_transfer_user.go

@@ -8,10 +8,10 @@ import (
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/transfer"
-
 	"github.com/hashicorp/terraform-plugin-sdk/helper/resource"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/schema"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/validation"
+	"github.com/terraform-providers/terraform-provider-aws/aws/internal/keyvaluetags"
 )
 
 func resourceAwsTransferUser() *schema.Resource {
@@ -89,7 +89,7 @@ func resourceAwsTransferUserCreate(d *schema.ResourceData, meta interface{}) err
 	}
 
 	if attr, ok := d.GetOk("tags"); ok {
-		createOpts.Tags = tagsFromMapTransfer(attr.(map[string]interface{}))
+		createOpts.Tags = keyvaluetags.New(attr.(map[string]interface{})).IgnoreAws().TransferTags()
 	}
 
 	log.Printf("[DEBUG] Create Transfer User Option: %#v", createOpts)
@@ -135,7 +135,7 @@ func resourceAwsTransferUserRead(d *schema.ResourceData, meta interface{}) error
 	d.Set("policy", resp.User.Policy)
 	d.Set("role", resp.User.Role)
 
-	if err := d.Set("tags", tagsToMapTransfer(resp.User.Tags)); err != nil {
+	if err := d.Set("tags", keyvaluetags.TransferKeyValueTags(resp.User.Tags).IgnoreAws().Map()); err != nil {
 		return fmt.Errorf("Error setting tags: %s", err)
 	}
 	return nil
@@ -181,8 +181,11 @@ func resourceAwsTransferUserUpdate(d *schema.ResourceData, meta interface{}) err
 		}
 	}
 
-	if err := setTagsTransfer(conn, d); err != nil {
-		return fmt.Errorf("Error update tags: %s", err)
+	if d.HasChange("tags") {
+		o, n := d.GetChange("tags")
+		if err := keyvaluetags.TransferUpdateTags(conn, d.Get("arn").(string), o, n); err != nil {
+			return fmt.Errorf("error updating tags: %s", err)
+		}
 	}
 
 	return resourceAwsTransferUserRead(d, meta)