[nrk] please add support for NRK Radio podcasts

[barsnick]

Checklist

• I'm reporting a new site support request
• I've verified that I'm running youtube-dl version 2020.12.31
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that none of provided URLs violate any copyrights
• I've searched the bugtracker for similar site support requests including closed ones

Example URLs

• Single podcast, no thumbnail, redirect from /podcast to /podkast: https://radio.nrk.no/podcast/ulrikkes_univers/nrkno-poddkast-26588-134079-05042018030000
• Single podcast, spurious spaces in the title: https://radio.nrk.no/podkast/hele_historien/sesong/bortfoert-i-bergen/l_774d1a2c-7aa7-4965-8d1a-2c7aa7d9652c
• Single podcast, shorter URL: https://radio.nrk.no/podkast/loerdagsraadet/l_1985e1f5-6c6f-4047-85e1-f56c6f3047aa
• Series: https://radio.nrk.no/podkast/loerdagsraadet
• Series with season: https://radio.nrk.no/podkast/hele_historien/sesong/diagnose-kverulant

Description

Norway's national radio broadcaster NRK also provides podcasts on its website. It uses some of the infrastructure from the TV and radio programs (such as PSAPI hosts for metadata), but requires a different extractor, from what I can tell (different webpage, JSON, and so on).

A previously reported issue #16097 was closed by the reporter.

I have written an extractor which I have used successfully (and improved) for the last eight weeks, I will post a PR in a moment.