hlssink3: Support generating WebVTT HLS manifests

It would be nice to extend the hlssink3 element to add a new pad to accept application/x-subtitle-vtt-fragmented buffers and then be able to generate WebVTT HLS manifests. Eg.

#EXTM3U
#EXT-X-TARGETDURATION:6
#EXT-X-VERSION:3
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-PLAYLIST-TYPE:VOD
#EXTINF:6.00000,	
fileSequence0.webvtt
#EXTINF:6.00000,	
fileSequence1.webvtt
#EXTINF:6.00000,	
fileSequence2.webvtt
#EXTINF:6.00000,	
fileSequence3.webvtt
#EXTINF:6.00000,	
fileSequence4.webvtt
#EXTINF:6.00000,	
fileSequence5.webvtt
#EXTINF:6.00000,	
fileSequence6.webvtt
#EXTINF:6.00000,

I imagine that adding this capability should enable the use of the closedcaption/jsontovtt element in combination with the hlssink3 element. One question I have is, shall the hlssink3 support one manifest type per instantiation, or should we be able to just send all caps (audio, video, and captions) to a single instance of hlssink3?

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