feat: add encrypted and non-blocking cancellation

feat: add encrypted and non-blocking cancellation

We introduce Connection.cancel_safe() which uses the encrypted and
non-blocking libpq cancellation API available with PostgreSQL 17. As a
non-blocking entry point, cancel_safe() delegates to a generator function,
namely _cancel_gen(). If the libpq version is too old, the method raises
a NotSupportedError.

CTRL+C handling (in Connection.wait() or Cursor.stream()) also uses the
non-blocking cancellation but falls back to old method if the former is
not supported.

The behavior of cancel() method (either on Connection or
AsyncConnection) is kept unchanged to use the old cancellation API.