Move evaluatable definitions out of EFCore.PG and into the plugin

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Hi @kinosang, hope you're well and that the plugin is in good shape - saw you released 3.1.0 a few weeks ago, congrats :)

For 5.0, EF Core finally added support for evaluatable filters in plugins (dotnet/efcore#13454), so we can now remove the string-based hack we have in the EFCore.PG provider to support the PGroonga plugin. Here's a PR which does this for the NodaTime type, which you should be able to more or less copy-paste. Once that's done I'll remove the EFCore.PG hack.