Video related primitives should be provided in scraperlist

We are publishing more and more ZIM files with videos using many different scrapers.

Do do that we mainly:

• Re-encode videos/audio streams
• Handle sub-titles
• Display all of this using video.js

For the moment many of these pieces of video related functions are distributed in different places (in the scraper relying on them).

At least to me, this is:

• difficult to understand which scraper is at which status related to the development of these pieces of code
• Fixes will have to be requested/tracked in each scraper
• Releases are also de-facto made at very different times

I'm not prescriptive about the exact solution, but I believe we should try to consolidate this at one place.

Inizia facendo un inventario delle funzionalità relative ai video attualmente distribuite tra gli scraper che usano python-scraperlib, inclusi la ricodifica di video/audio, i sottotitoli e la visualizzazione con video.js. Definisci un unico approccio al consolidamento e identifica cosa consumerebbe ogni scraper da esso. Il lavoro è completato quando queste primitive vengono mantenute e rilasciate da un unico punto condiviso.