Add backend requirements to program __str__

[kt474]

Summary

Details and comments

Fixes Qiskit/qiskit-ibm-provider#198

Was only able to find input_allowed and min_number_qubits as properties of backend_requirements - are there others?

[rathishcholarajan]

1. Could you paste the output of printing a sample program by running this code?
2. Could we add some tests?

[kt474]

service.pprint_programs(detailed=True)

Example outputs

qiskit-test:
  Name: qiskit-test
  Description: Qiskit test program
  Creation date: 2021-10-10T18:48:27Z
  Update date: 2021-10-10T18:48:27Z
  Max execution time: 600
  Backend requirements:
    none
  Input parameters:
    none
  Interim results:
    none
  Returns:
    none

qasm3-runner:
  Name: qasm3-runner
  Description: A runtime program that takes one or more circuits, compiles them, converts them to OpenQASM3, and executes them. The input can also be one or more OpenQASM3 strings. Note that only one circuit/string can be passed if using a simulator, and this program can only run on a backend that supports OpenQASM3. This program is currently in beta mode and will likely be updated frequently.
  Creation date: 2021-11-01T17:57:48.28061Z
  Update date: 2021-12-10T23:33:14.828172Z
  Max execution time: 14400
  Backend requirements:
    Supported features: qasm3
  Input parameters:
    Properties:
    ... 

[rathishcholarajan]

LGTM!