add user_id as input to header too for http batch V2 API

[giorgosHadji]

add user_id as input to header too for http batch V2 API, follow up from #95.

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, user_id="MY_USER_ID")
        print(result)

asyncio.run(main())

[dumitrugutu]

LGTM