Add support for Identity Management Plugin (#14913)

- Adds an STS API `AssumeRoleWithCustomToken` that can be used to 
  authenticate via the Id. Mgmt. Plugin.
- Adds a sample identity manager plugin implementation
- Add doc for plugin and STS API
- Add an example program using go SDK for AssumeRoleWithCustomToken

cmd/sts-errors.go

@@ -96,6 +96,7 @@ const (
 	ErrSTSInsecureConnection
 	ErrSTSInvalidClientCertificate
 	ErrSTSNotInitialized
+	ErrSTSUpstreamError
 	ErrSTSInternalError
 )
 
@@ -162,6 +163,11 @@ var stsErrCodes = stsErrorCodeMap{
 		Description:    "STS API not initialized, please try again.",
 		HTTPStatusCode: http.StatusServiceUnavailable,
 	},
+	ErrSTSUpstreamError: {
+		Code:           "InternalError",
+		Description:    "An upstream service required for this operation failed - please try again or contact an administrator.",
+		HTTPStatusCode: http.StatusInternalServerError,
+	},
 	ErrSTSInternalError: {
 		Code:           "InternalError",
 		Description:    "We encountered an internal error generating credentials, please try again.",