Add support for filtering allowed values for predefined lists.

[Etroid]

By default, when using a function (e.g. states, timezones etc...) to generate a list of allowed values, ALL options will always be available to the editor to choose from. There is no configuration to filter the list down to a set of allowed values. However, there may be times where you don't want to editors to be able to choose every possible option.

Example: Let's say we create an option field which has a function to list all the available content types on the site. However, we only want this editors to choose content type A, C and D, not B. Currently there is no way to configure it as such.

This PR gives admins the option to filter down this if desired turning it into a hardcoded list of options instead of the dynamic function. If not filtered down, it will use the dynamic function.