Issue #3570073: Add SRT caption fallback with server-side VTT conversion

Johan den Hollanderrequested to merge
issue/rijksvideo-3570073:3570073-srt-caption-fallback into 2.x
  • Add SRT fallback when VTT caption is not available
  • Convert SRT to VTT format server-side for native browser support
  • Add downloadCaptionFile(), downloadSrtAsVtt(), and convertSrtToVtt() methods

Closes #3570073

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