postgres: SSL mode creates temporary files in certain situations, we should not do that

[gabor]

the postgres go library we currently use ( lib/pq ) requires the TLS certificates to be files on the disk.
so, when the user simply pastes the certificates into the datasource config page, this won't work.
to fix this,we create temporary files on the disk.

we should find another approach, where temporary files are not needed, because the current approach makes it hard to do improvements, like multi-tenancy.

what we tried and did not work:

• lib/pq supports the sslinline flag, that should allow us to directly insert the certificate-content into the connection string. unfortunately, currently, if you specify sslinline, it is mandatory to have client certificates configured ( ssl: allow sslinline without client certs lib/pq#1140 )

what should work, is #87037