[requestforcomments] new extractor for requestforcomments podcast

[kayb94]

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What is the purpose of your pull request?

• New extractor

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Description of your pull request and other information

RequestForComments is another podcast, which also uses more or less the same webpage as the Metaebene podcasts (see #15872), but from another producer. This is an extractor for the page (requestforcomments.de). It is not associated with the IETF.
I decided to make two seperate extractors because:

• I did not want RequestForComments to appear as one of the podcasts of the Metaebene producer, because that would be wrong
• I did not find a way, to use the logic of MetaebeneIE in RequestForCommentsIE without instantating an object of it or stuff.

As the extraction is very simple, I think it is okay, to keep them as two. This might also help in the future, if either one of the Metaebene pages or the RequestForComments page should change.