Provide an IsUnicode API

[rowanmiller]

As pointed out in #3362 we have removed this API which forces you to specify a provider specific type in cases where you would not have needed to in EF6. We should add this into EF7.

[joshmouch]

I think IsFixedLength() is probably the other one that is agnostic yet can have an easy fallback if not supported by the provider. I use that because I override the entities in Microsoft Identity 2.0, and it uses that quite a bit.

[bricelam]

Note to implementer: Don't forget to diff the metadata in MigrationsModelDiffer.Diff(IProperty, IProperty).

[lajones]

Note to implementer: when we do this we'll need to update scaffolding to output IsUnicode(false) for varchar columns instead of outputting HasColumnType("varchar(x)") as we do now. See discussion in #4312.

[divega]

Cleaning up milestone so that we can see this in triage.