feat: assume role provider credentials (#28)

* feat: assume role provider credentials

This commit updates the awsutil dependency to its latest version, which
supports the capability of using dynamic credentials. The storage service
was refactored to make static credentials optional. The credentials pkg
was refactored to support several attribute fields used for the assume
role provider. The host service still enforces the use of static
credentials.

plugin/service/storage/README.md

@@ -42,7 +42,7 @@ Example policy:
 ```
 
 * `iam:GetUser`, `iam:CreateAccessKey`, and `iam:DeleteAccessKey`, configured to
-  the IAM user to allow credential rotation. Example policy:
+  the IAM user to allow static credential rotation. Example policy:
 
 ```
 {
@@ -66,18 +66,24 @@ Example policy:
 The following `attributes` are valid on an AWS storage bucket resource:
 
 - `disable_credential_rotation` (bool): If `true`, credential rotation will not
-  be performed. See the [Credential Rotation](../../../README.md#credential-rotation) section
-  above.
+  be performed.
 - `region` (string): The region to configure the storage bucket for.
 - `endpoint-url` (string): The endpoint to configure the storage.
+- `role_arn` (string): The role arn configured for the assume role provider.
+- `role_external_id` (string):  The external id configured for the assume role provider.
+- `role_session_name` (string): The session name configured for the assume role provider.
+- `role_tags` (object): The key-value pair tags configured for the assume role provider.
 
 An example of how to utilize the [endpoint attribute](https://docs.aws.amazon.com/vpc/latest/privatelink/vpc-endpoints-s3.html)
+See the [Credential Rotation](../../../README.md#credential-rotation) readme for more information.
 
 ### Secrets
 
-The following `secrets` are valid on an AWS storage bucket resource:
+The following `secrets` are optional on an AWS storage bucket resource:
 
 - `access_key_id` (string): The access key ID for the IAM user to use with this
   storage bucket.
 - `secret_access_key` (string): The secret access key for the IAM user to use
-  with this storage bucket.
+  with this storage bucket.
+
+See the [Credential Rotation](../../../README.md#credential-rotation) readme for more information.