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/host/README.md

@@ -105,18 +105,19 @@ a configured IAM user for this provider:
 The following `attributes` are valid on an AWS host catalog 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. See the [Credential Rotation](../../../README.md#credential-rotation) readme for more information.
 - `region` (string): The region to configure the host catalog for. All host sets
   in this catalog will be configured for this region.
 
-The following `secrets` are valid on an AWS host catalog resource:
+The following `secrets` are required on an AWS host catalog resource:
 
 - `access_key_id` (string): The access key ID for the IAM user to use with this
   host catalog.
 - `secret_access_key` (string): The secret access key for the IAM user to use
   with this host catalog.
 
+See the [Credential Rotation](../../../README.md#credential-rotation) readme for more information.
+
 ### Host Set
 
 The following attributes are valid on an AWS host Set resource: