It should be possible to get the two independent readout error rates from a Target object

[nonhermitian]

What is the expected feature or enhancement?

It is possible to grab the two independent readout error rates from the backend.properties:

props = backend.properties()
P01 = props.qubit_property(0, 'prob_meas0_prep1')[0]
P10 = props.qubit_property(0, 'prob_meas1_prep0')[0]

It would be nice if this information was included in the backend.target so that (at least in this instance) users can get all information from a single object.

Acceptance criteria
The 2 readout error rates are in the Target

[mtreinish]

For reference the pattern to do this is to make a custom InstructionProperties subclass that extends it with custom properties for a given backend. There is an example of how to do this in qiskit's unit tests here:

https://github.com/Qiskit/qiskit/blob/fe275a0f15132073c6b5afb0c4be8bcc351479cb/test/python/transpiler/test_target.py#L1015-L1028

This is similar to what is already done for QubitProperties here:

qiskit-ibm-runtime/qiskit_ibm_runtime/ibm_qubit_properties.py

Line 18 in 4b938ed

class IBMQubitProperties(QubitProperties):

which adds anharmonicity and a boolean for the operational state for each qubit in the target.