bench(asr-worker): perform load testing

[ClemDoum]

In order to avoid discovering our system at large scale the first time we'll need it, perform load testing at moderately large scale to assess:

• does the service scale properly ?
• what's the end-to-end RTF (with temporal overhead, preprocessing, machine provisioning and so on): "how many hours does it take to transcribe end-to-end X hours of audio"
• how much does it cost ?
• is worker startup fast enough for small volumes of documents ?

[ClemDoum]

Potential priority for sprint 50: P0 (as it's been ongoing for past sprints)