code: @Beta
      public MSICredentials withIdentityId(String identityId) {
          this.identityId = identityId;
          this.clientId = null;
          this.objectId = null;
          return this;
      }
docs: Specifies the ARM resource id of the user assigned managed service identity resource that
  should be used to retrieve the access token.
  
  @param identityId the ARM resource id of the user assigned identity resource
  @return MSICredentials