Feature Request: List custom field type

[EweSparky]

(split out from #1206)

It'd be nice to have a list custom field type, where a dynamic drop-down list is used and populated with existing entries. New entries can be added by typing into the field and saving, rather than selecting an existing item. This sort of thing would be useful for where the values can change, but the phrasing is always the same. For example:

collection: Platforms
custom_list_name: platform_name
custom_description: Which platform or service did I buy this on or stream it on?
custom_list_entries: [
  Amazon Prime,
  Apple TV,
  Netflix
]

collection: Recommended
custom_list_name: recommender_name
custom_description: Who recommended it to me?
custom_list_entries: [
  Bob,
  Mary
]

then later on, I might search for a movie available on Shudder recommended by John. I could add the movie to the Platforms collection and type in Shudder for the platform name, and add it to the Recommended collection and type in John for who recommended it to me. If John recommended a second movie available on Shudder some other day, I would be able to select Shudder and John from the respective collections because the drop-down menu of list items looks up all unique values and displays them (alphabetically).