option to pass managed identity client id to AD authentication (#45)

* https://github.com/Azure/azure-amqp-common-go/issues/44 option to use managed identity to be used for MSI authentication

* https://github.com/Azure/azure-amqp-common-go/issues/44 option to use managed identity to be used for MSI authentication

* fix formatting

* rename method

Co-authored-by: Muthusamy Saravanan <saravanan.muthusamy@bluvision.com>
Co-authored-by: Joel Hendrix <jhendrix@microsoft.com>

aad/jwt.go

@@ -188,7 +188,7 @@ func (c *TokenProviderConfiguration) NewServicePrincipalToken() (*adal.ServicePr
 	if err != nil {
 		return nil, err
 	}
-	spToken, err := adal.NewServicePrincipalTokenFromMSI(msiEndpoint, c.ResourceURI)
+	spToken, err := c.getMSIToken(msiEndpoint)
 	if err != nil {
 		return nil, fmt.Errorf("failed to get oauth token from MSI: %v", err)
 	}
@@ -198,6 +198,13 @@ func (c *TokenProviderConfiguration) NewServicePrincipalToken() (*adal.ServicePr
 	return spToken, nil
 }
 
+func (c *TokenProviderConfiguration) getMSIToken(msiEndpoint string) (*adal.ServicePrincipalToken, error) {
+	if c.ClientID == "" {
+		return adal.NewServicePrincipalTokenFromMSI(msiEndpoint, c.ResourceURI)
+	}
+	return adal.NewServicePrincipalTokenFromMSIWithUserAssignedID(msiEndpoint, c.ResourceURI, c.ClientID)
+}
+
 // GetToken gets a CBS JWT
 func (t *TokenProvider) GetToken(audience string) (*auth.Token, error) {
 	token := t.tokenProvider.Token()