Move enum constraints convention out of EF Core to a plugin

[roji]

#15411 added a convention to create database check constraints for enums. We've decided this would be more appropriate for an external plugin for the following reasons:

• Check constraints have a perf impact, and if the database is only accessed by EF they aren't needed. A plugin would provide an explicit opt-in for this.
• The current convention is SQL Server-specific, but could be used on other databases as well (Move enum check constraints convention to relational? #20878).

[roji]

PR for the plugin: efcore/EFCore.CheckConstraints#1

[smitpatel]

@roji - I think you already completed this work item.

[roji]

Thanks @smitpatel.