Batch support for the next gen model

[rakeshv247]

This change adds

• Support for the new next-gen model melia-1.

• Support for the language hints feature, which is applicable only for the batch next-gen models. The language_hints info will be a new field in the transcription_config. For example:

  {
      "language": "multi",
      "language_hints": ["en", "es"],
      "language_hints_strict": True
  }
  

• Support new fields in the metadata.language_pack_info from transcript.json-v2 results.

[giorgosHadji]

Have you tested this with an actual worker?

[giorgosHadji]

LGTM in general

[rakeshv247]

Have you tested this with an actual worker?

Tested the changes with omni and non-omni workers in a SaaS dev environment.

[J-Jaywalker]

Reviewed, left a couple of minor comments.

[mnemitz]

Looks good! Maybe we should wait until melia-1 lands before releasing though.