Provide methods to update list settings and add columns after it was created

[OndrejValenta]

Provide methods to update field settings and list settings to CSOMOperation.

op.GetList("Test list")
    .ModifyField<NumberField>("NumberField1", field => {
              field.Required = false; // property is made up use the proper one
              field.DefaultValue = 3.14; // property is made up use the proper one
     }

ModifyField method should be generic, if possible, to provide user with proper field properties without a need to convert the object to specific field type.