[characterization] Add support for discretely coding combined formats with IVAS

Some of the P800 configurations for characterization require OMASA/OSBA modes to be compared to discretely combined IVAS-ISM + IVAS-SBA/MASA.

This is not supported in the scripts and should be implemented, e.g.:

input:
    fmt: "ISM4SBA3"
    fs: 48000
    aligned_to:
        len: 20
conditions_to_generate:
  c123:
      type: ivas_combined # needs implementation
      bitrates: [ 32000, 32000 ]
  c456:
      type: ivas
      bitrates:
          - 64000