python: Use direct TLS connections for psycopg2

[danielfrankcom]

This PR modifies the psycopg2 sample to set sslnegotiation=direct as per #150.

It does this conditionally based on the underlying libpq version, since the same psycopg2 version can be used with multiple versions of libpq. I verified in the release notes here that sslnegotiation has been available since the 17.0 release and was not added as a minor version bump.

At the time of writing, the psycopg2-binary package includes libpq 16.0, which doesn't support the sslnegotiation parameter. I manually checked a version of psycopg2 compiled against libpq 17.4 and the direct TLS connection worked properly. I have kept the sample dependency on the pre-built binary, as it's more approachable in terms of the system requirement for running the sample. If/when psycopg2-binary is updated to include libpq 17.0+, it will start using sslnegotiation without any changes here.

I added a section to the README.md file which describes more about the TLS configuration, and calls out that direct TLS connections are a new feature and may not be supported by all driver versions. This section was already reviewed in #160, and will be added to all samples which support direct TLS connections.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT-0 license.

[leszek-bq]

LGTM