Wire TlsConfig into ConnectionBuilder and Config

[StefanSteiner]

Summary

TlsConfig exists in hyperdb-api-core but is not reachable through the public Connection / AsyncConnection / ConnectionBuilder surface. There is no with_tls() / tls() method on any of them. TLS code is built and tested at the lower layer but has no on-ramp from the high-level API.

Current state

$ grep -rn "fn with_tls\|fn tls\|TlsConfig\|tls_config" \
    hyperdb-api/src/connection_builder.rs \
    hyperdb-api-core/src/config.rs
# (no output)

The original gap analysis (§7 of docs/RUST_API_GAP_ANALYSIS.md in the predecessor repo) called this out:

TlsConfig exists with a full builder. But the module doc says "TODO: with_tls not yet on Config" — i.e. there is no way to pass a TlsConfig to Client::connect(&Config). Config has no tls field/method. TLS code exists but isn't reachable through the public Connection/AsyncConnection or ConnectionBuilder.

Proposed work

• Add a tls(TlsConfig) -> Self method on Config in hyperdb-api-core (or whichever struct Connection consumes for its TLS settings).
• Add a tls(TlsConfig) -> Self method on ConnectionBuilder and its async counterparts.
• Wire the field through to the actual Client::connect call so it has effect.
• Add an integration test that connects to a TLS-enabled Hyper endpoint via the high-level API.
• Add an example under hyperdb-api/examples/ demonstrating TLS configuration.
• Update the rustdoc on ConnectionBuilder to mention TLS.

Backwards compatibility

Purely additive — new methods, no changes to existing signatures.

Note on cert verification

Whatever default TlsConfig produces today should remain the default. This issue is about exposing TLS, not redesigning the security defaults.