Enable value conversion support for identity/sequence properties

[lauxjpn]

Allows to use value converters for identity and sequence columns, within the limitations already established (must be a non-floating point numerical type on the provider side).

Implements PomeloFoundation/Pomelo.EntityFrameworkCore.MySql#1118 for SQL Server.

I added an explicit .UnwrapNullableType() call, so that a decimal? would behave the same way as an int? (both are not really supported for identity columns by SQL Server, but they should at least be handled in the same way before SQL Server finally returns an error, and should not lead to different behavior in EF Core).
It would probably be cleaner to check for non-nullable types only in general instead.

Related to #11597 and npgsql/efcore.pg#1439

I don't see any issues with this PR in regards to #11597.
If there are, feel free to add a test or two to demonstrate those.

[AndriySvyryd]

Could you also add a test (or modify an existing one) that actually inserts a row with Identity and a converter?

[AndriySvyryd]

If you remove the check for all types of store-generation, we'll also need tests for sequences, default values and computed columns. As well as a variety of store/client type combinations. This will also need a review from @ajcvickers when he's back, to make sure this is in-line with #11597 and #11970

[ajcvickers]

I think this is probably okay; I can't think of a way it will break after a more general solution. I'm inclined to take this; we will discuss again next triage.

[AndriySvyryd]

Thanks for your contribution!