add support for on-the-fly generation of WebVTT captions
use a KS with caption privilege for securing WebVTT captions

The KS of the playManifest may expire before the video is over. A KS
with a null role (allowing only 'always allowed actions') and a caption
privilege for the specific entry id is created instead
Merge pull request #109 from kaltura/IX-9.1.0-serve-webvtt-captions

Ix 9.1.0 serve webvtt captions