Add compatibility for Amazon Aurora DSQL

[GunboatDiplomat]

DSQL has a large subset of Postgres features. It's compatible with Npgsql as noted here
https://docs.aws.amazon.com/aurora-dsql/latest/userguide/SECTION_program-with-net.html

It would be nice if we could have a compatibility flag that makes EF avoid unsupported SQL. It's possible that everything already works, but I can not confirm that for all cases, this is the list of unsupported Postgres features:
https://docs.aws.amazon.com/aurora-dsql/latest/userguide/working-with-postgresql-compatibility-unsupported-features.html

[roji]

@GunboatDiplomat I wouldn't be against a config switch that tells the provider that Aurora is being targeted, which would affect the kind of SQL that gets generated. But somehow would have to put in the work to identify exactly what needs to be done, test things, etc. etc.

In addition, for e.g. foreign keys (which aren't supported), dotnet/efcore#15854 is likely needed. There may be other things that would require EF-side changes.

[GunboatDiplomat]

Thanks @roji for the update. If I understood correctly, foreign keys would only be an issue with navigation properties and code migrations in EF, so that's not a blocker (at least it isn't for my use case). I guess the next step would be to run all tests against Aurora DSQL and see how compatible the current situation is.