Expose scheduling passes via a scheduling plugin.

[mtreinish]

What is the expected enhancement?

Right now there are several transpiler scheduling passes in the ibm provider package here: https://github.com/Qiskit/qiskit-ibm-provider/tree/main/qiskit_ibm_provider/transpiler/passes/scheduling but to use these you have to manually construct a PassManager. While using the staged pass manager makes this easier to integrate into a preset pass manager. For users it would be really nice if this was exposed via the plugin interface (https://qiskit.org/documentation/apidoc/transpiler_plugins.html) and have multiple scheduling plugins so that users can just do:

transpile(qc, backend, scheduling_method="ibm_alap_dd")

This still leaves the flexibility for power users that want to experiment with different DD sequences or other parameters to use the existing interface. But having a plugin with sane defaults will make the use of the scheduling passes more accessible.