m3u8-parser, EXT-X-KEY URI handling

[itsjamie]

Is your feature request related to a problem? Please describe.

Currently, the m3u8-parser for handling EXT-X-KEY doesn't handle keys that aren't URLs.

I believe absolute versus relative URLs are handled via the url-toolkit dependency we are using, but we need to be able to detect when we aren't dealing with a URL path, and instead a URI, so that we can flow down the alternative path of not using the url-toolkit.

Describe the solution you'd like

The TODO noted below needs to be handled so we can work towards manifest signalled DRM in the data model of hls.js.

Additional context

https://github.com/video-dev/hls.js/pull/2735/files#diff-c66c76a43ba1843baa5bdcec54b3bec2R328-R330