add EXT-X-KEY tag support to playlist parser

[Lysander66]

Media Segments MAY be encrypted.

Add support for EXT-X-KEY tag in m3u8 playlist parser.

Ref: https://datatracker.ietf.org/doc/html/rfc8216#section-4.3.2.4

[aler9]

Hello, thanks for this patch, just fix lint errors and add a couple of tests and it will be merged.

[Lysander66]

Hello, thanks for this patch, just fix lint errors and add a couple of tests and it will be merged.

Hi, I've fixed the lint errors and added the test cases for EXT-X-KEY tag with different scenarios.

[codecov]

Codecov Report

Attention: Patch coverage is 80.51948% with 15 lines in your changes missing coverage. Please review.

Project coverage is 73.27%. Comparing base (047a1d4) to head (2d8d50e).

Files with missing lines	Patch %	Lines
pkg/playlist/media_key.go	80.32%	8 Missing and 4 partials ⚠️
pkg/playlist/media.go	81.25%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   73.15%   73.27%   +0.11%     
==========================================
  Files          57       58       +1     
  Lines        3975     4052      +77     
==========================================
+ Hits         2908     2969      +61     
- Misses        870      882      +12     
- Partials      197      201       +4     

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