Support PDF and Audio xblocks

[satyamtg]

Currently, the scraper supports a limited set of xblocks, which are basically enough to scrape most of the courses, but we shall also support some other xblocks like the PDF and the audio xblock which are used sometimes. A list of all xblocks can be found here -
https://openedx.atlassian.net/wiki/spaces/COMM/pages/43385346/XBlocks+Directory

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.