[HLSTree] Add support to read subtitle codec

[CastagnaIT]

Description

Allow to read subtitile codec string from variant
and so avoid forcing webvtt codec

Motivation and context

on ISA Samples addon there is an example of HLS with ttml subtitle

HLS parser force webvtt so will result in corrupted subs

anyway with that sample stream the subtitles are broken after a video seek, this will not be fixed here

How has this been tested?

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly