Easier compilation to discrete basis sets

[Cryoris]

What should we add?

From discussions with @alexanderivrii: It would be nice if we could detect a discrete basis set in transpile or the preset PMs and then use a discrete basis synthesis, like the Solovay-Kitaev algorithm (or others in the future). Something like

transpile(circuit, basis_gates=["h", "t", "tdg", "cx"], translation_method="synthesis")

One way to do this could be to write a default plugin for discrete basis that takes n-qubits unitaries to U + {2q-gates in the basis gates} (e.g. using the existing unitary synthesis workflow) and then uses the Solovay-Kitaev algorithm to decompose the 1-qubit U gates.

It would be even nicer to skip the translation_method="synthesis" argument, but that might be necessary to distinguish the command from trying to use the basis translator (alternatively, we could auto-select based on the basis gates and users can force a behavior by setting this arg).

[jakelishman]

imo we should:

• introduce a new translation_method="default" plugin, so we're not tying the default to a concrete implementation, and we can have "default" include this circuit-based dispatch
• consider if we'll eventually need a whole other pipeline for discrete optimisation - it's going to matter a lot more when we need to target EC backends, and our current pipeline is pretty heavily assumed to be targetting a continuous basis.