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.

README.md

@@ -5,15 +5,32 @@ Boundary](https://www.boundaryproject.io/).
 
 ## Credential Rotation
 
-Although credentials are stored encrypted within Boundary, by default this
+This is the following priority for the credential chain:
+static, assume role, environment variables.
+
+### Static Credentials
+
+Although static credentials are stored encrypted within Boundary, by default this
 plugin will attempt to rotate credentials when they are supplied through the
 `secrets` object. The given credentials will be used to create a new credential,
 and then the given credential will be revoked. In this way, after rotation,
-only Boundary knows the client secret in use by this plugin.
+only Boundary knows the client secret in use by this plugin. More information
+about AWS static credentials can be found [here](https://docs.aws.amazon.com/sdkref/latest/guide/feature-static-credentials.html).
 
 Credential rotation can be turned off by setting the 
 `disable_credential_rotation` attribute to `true`.
 
+### Assume Role Credentials
+
+This plugin will attempt to assume a role when a `role_arn` is supplied through the
+`attributes` object. More information about assume an AWS role can be found [here](https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html). This feature only works when the plugin is running on a self managed Boundary [worker](https://developer.hashicorp.com/boundary/tutorials/hcp-administration/hcp-manage-workers).
+
+### Environment Credentials
+
+This plugin will attempt to retrieve credentials from environment variables. More
+information about environment variables for AWS credentials can be found [here](https://docs.aws.amazon.com/sdkref/latest/guide/feature-static-credentials.html). This feature only works when the plugin is running on
+a self managed Boundary [worker](https://developer.hashicorp.com/boundary/tutorials/hcp-administration/hcp-manage-workers).
+
 ## Dynamic Hosts
 
 This plugin supports dynamically sourcing hosts from Amazon EC2.