Update circuit.delay to accept parameterized duration (Fix #6029)

[itoko]

Summary

Updates circuit.Delayto be able to accept a parameterized duration as well as a fixed (integer/float) duration.
Fixes #6029.

Details and comments

Now you can specify a duration of delay as a ParameterExpression object when appending it to a circuit, and assign some value to the parameter afterward.

idle_dur = Parameter('t')
qc = QuantumCircuit(1, 1)
qc.x(0)
qc.delay(idle_dur, 0, 'us')
qc.measure(0, 0)
print(qc)  # parameterized delay in us (micro seconds)

# assign before transpilation
assigned = qc.assign_parameters({idle_dur: 0.1})
print(assigned)  # delay in us
transpiled = transpile(assigned, some_backend_with_dt)
print(transpiled)  # delay in dt

# assign after transpilation
transpiled = transpile(qc, some_backend_with_dt)
print(transpiled)  # parameterized delay in dt
assigned = transpiled.assign_parameters({idle_dur: 0.1})  # parameter's unit is kept in us
print(assigned)  # delay in dt

[kdk]

Thanks @itoko , this looks great.