add support for requesting parallel engines in a http batch job

[giorgosHadji]

To specify the number of ASR engines a job on http batch worker(on Prem container) can use we added a new header called X-SM-Processing-Data. Inside the header we can have a json dictionary, with a key called parallel_engines where you can specify the number of engines you want to use.

Usage with:

from speechmatics.batch import AsyncClient

async def main():
    async with AsyncClient(url='http://localhost:9002/v2') as client:
        result = await client.transcribe(sys.argv[1],parallel_engines=2)
        print(result)

asyncio.run(main())

[dumitrugutu]

LGTM