Make DiscriminatorValue object instead of string

[rmja]

The DiscriminatorValue is currently a string, but should be object to allow for e.g. enums to discriminate derived entity types. See e.g. #2057 where a naiive try results in invalid SQL for enum.

[anpete]

I think this is a hangover from when annotations values could only be strings. Should be a small'ish change. Sending back to triage.