Update VideoConfigService get_transcript method

[farhan]

Description

This moves edx-platform-specific logic out of the VideoBlock, in preparation for the VideoBlock extraction: https://github.com/openedx/edx-platform/issues/36282

Testing instructions

Testing has been done on the sandbox created within this PR.

1. I have created youtube video xblock on studio/content-library
2. I have uploaded the english/french transcripts on these video xblocks
3. Transcripts feature is working fine on studio/content-library video xblocks
4. Transcripts feature is working fine on lms for both course and library added video xblock in the course videos.

Note: Bumper video testing can't be done as not frontend exist for this feature. It seems un-used feature for some time.

Acceptance Criteria:

• Transcripts feature should work for the video xblocks.
• There should be no logic change.

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