Allow out-of-band text tracks to source kind, label, language from the text track format

[zcorpan]

What is the issue with the HTML Standard?

See discussion in https://github.com/w3c/webvtt/pull/523/files#r2348659042

Right now the sourcing of out-of-band text tracks is specified in HTML to use the track elements attributes, or fall back to the empty string or a UA-generated string (for label). If the WebVTT change moves forward to allow specifying these metadata in the file, HTML needs some changes to https://html.spec.whatwg.org/#sourcing-out-of-band-text-tracks to use that metadata when the track element's attributes are absent.

cc @whatwg/media

[cookiecrook]

Update: WebVTT PR #523 is now superseded by WebVTT #548 due to a Working Group decision that the ATTRIBUTES block was not necessary, and instead these key/value pairs should be formalized in the main file header.