rust: Add direct TLS negotiation support to SQLx sample

**Is your feature request related to a problem? Please describe.**
The SQLx Rust library has a pending PR that adds support for PostgreSQL 17's direct TLS negotiation feature. This feature improves connection performance and compatibility with certain proxies, but our sample code hasn't been updated to take advantage of it yet.

**Describe the solution you'd like**
Once the upstream PR is merged, modify the SQLx sample to add support for the `sslnegotiation` connection parameter. This should demonstrate how to configure direct TLS negotiation for PostgreSQL 17+ servers.

**Describe alternatives you've considered**
- None available.

**Additional context**
This feature was originally requested in #150 and has been broken out given many drivers already support this functionality.

The feature implementation is pending review from the maintainers in PR https://github.com/launchbadge/sqlx/pull/3879.

Wait for the upstream SQLx PR https://github.com/launchbadge/sqlx/pull/3879 to merge, then locate the SQLx sample and inspect its connection configuration. Add the `sslnegotiation` parameter to demonstrate direct TLS negotiation for PostgreSQL 17+ servers, and verify the sample configuration works.

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