SQL Server: introduce UseUtf8() sugar to configure UTF8 properties

#25798 added the ability to do `HasColumnType("varchar(max)").IsUnicode(true)`, which configures a UTF8 property on SQL Server. We could also introduce a UseUtf8() sugar (or UseUTF8() 🔥🔥🔥).

If we add it, you would be able to pass a non-UTF8 collation which is problematic. Or we'd validate the `_UTF8` postfix, which would encode the SQL Server convention into EF Core - not ideal.

Note also that UseUtf8() would require the collation to be passed in, but that's only actually necessary if the user is using migrations.