add subtitle parameter into x-callback-url

Checklist

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which cleans up / improves existing functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

add an parameter SUB on x-callback-url to push subtitle from an url callback now can be:

vlc-x-callback://x-callback-url/ACTION?url=...&sub=...&PARAMETER=...

if you do not want to use external subtitle then still using old form:

vlc-x-callback://x-callback-url/ACTION?url=...&PARAMETER=...

already test and work fine with SRT subtitle encode UTF-8

Actions: stream: VLC plays the stream provided by the URL parameter, if SUB parameter is valid then play with external subtitle