Allow and document usage of getFetchClient and youtubePlayerReq

[unixfox]

Invidious instance maintainers have different ways for rotating the IP address of their server.

Some use socks proxies, other uses openvpn servers and more.

Allow and document the ability to write some custom code that work in harmony with the videoplayback proxy and the video streams URL retrieval from YouTube servers.

Example: Allowing them to directly act on banned IP addresses and keeping existing video streams to work with existing IP addresses so that those streams are not cut by the IP rotation.

---

https://github.com/iv-org/invidious-companion/blob/master/src/lib/helpers/youtubePlayerHandling.ts#L4-L6
https://github.com/iv-org/invidious-companion/blob/master/src/main.ts#L6

---

Related to #5