Expose SQLite extension loading on driver connections

[rriski]

libsql_load_extension is available in the native API, but Go callers cannot use it on a pooled connection. This exposes a small LoadExtension method on the driver connection so applications can initialize each connection through sql.Conn.Raw or a connector hook.

The test checks the missing-extension error path and, when LIBSQL_TEST_EXTENSION_PATH is set, loads an extension and calls its function. Validated locally with sqlean-time 0.28.4 on macOS ARM64:

LIBSQL_TEST_EXTENSION_PATH=/tmp/sqlite-nanoclock-sqlean/time.dylib go test -run '^TestLoadExtension$' -count=1 .

AI disclosure: codex made this

Ref: #34