[PostgreSQL] Add support for AT TIME ZONE

[fabio-stein]

I have a table with data stored in UTC timestamptz, I want to select and group by date using a specific timezone on each execution.
As far as I did read, using the operation "AT TIME ZONE" in Postgres seems to be the most ideal solution.
I see that for SQL Server, EF already have the function implemented in #26199

Can we have EF.Functions.AtTimeZone used in PostgreSQL?

[roji]

Have you tried using TimeZoneInfo.ConvertTimeBySystemTimeZoneId() as listed in the docs? Did you run into any issues?

[roji]

Note also that this is the EF repo - Npgsql-specific issues belong in https://github.com/npgsql/efcore.pg. Am going to close this as external, but if you can't make it work with ConvertTimeBySystemTimeZoneId() please open an issue over there with a minimal, runnable repro.