qt6: add postgresql

[sabelka]

Enable openssl and the PostgreSQL QSql plugin for qt6, like it is also in qt5
fixes #2950

[mabrand]

Qt6 supports Window's native schannel. Is there a strong case for building with openssl support too, given the extra dependencies?

[sabelka]

Actually, I wasn't aware of Qt's schannel support on Windows. I just noticed, that QNetworkManager was not able do establish https connections, and enabling openssl for Qt fixed my problem. But I quess schannel would be fine, too.
However, it looks like it is not enabled by default (or I did something wrong in my test).
So, I'd be fine with any ssl solution.

[mabrand]

I think schannel is enabled. At least it works for me.

If we're going to enable the postgresql driver, that's going to drag in openssl anyway. Hmm.
Maybe you can do some checks with building support for both schannel and openssl and choosing the desired provider at runtime, just to make sure it works.

[sabelka]

I'll try and let you know!

[stefanha]

I've been using Qt6 QNetworkRequest successfully for HTTPS without explicitly enabling OpenSSL.

[sabelka]

Yes SSL works also without OpenSSL! (I had a mistake in my packager script, so the schannel plugin was in the wrong directory). So you are absolutely right, we don't need it on Windows.
So, I've modified this PR to include PostgreSQL only.

[sabelka]

Maybe you can do some checks with building support for both schannel and openssl and choosing the
desired provider at runtime, just to make sure it works.

It is possible to build both plugins, but I think it is not easily possible to choose the back end at runtime.
However, if you package just one of the two plugins with your application, Qt will use whatever it gets ;-)

[fiesh]

This adds a very heavy dependency to Qt permanently... I wouldn't want this for our build images, for example.