Port to Qoolqit with Pulser backends

[Yoric]

Once pasqal-io/qoolqit#67 has merged, Qoolqit will provide support for all Pulser backends + connections. Using these instead of custom backends will mean better compatibility with all architectures supported by Pulser.

To do this, we'll need to:

• behind the scenes, use Qoolqit's QuantumProgram instead of Pulse + Register + Detunings;
• behind the scenes, use Qoolqit's PulserBackend instead of _solver.backends;
• adapt the API to accept a Pulser Backend;
• adapt the tutorials.