BBC Maestro extractor

[zer0-delta]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

This adds the BBC Maestro extractor.
Apart from the course trailer, the course content requires a login.

When not being logged in, using the public course URL (not containing lesson), will download the course trailer video.

When logged in, course content URLs contain lesson and point to one of the lesson videos. They also contain the list of videos. There is no separate list URL.
The extractor would then treat the URL as a playlist.

Fixes #3395

Template

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check all of the following options that apply:

• I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)