[LinkedIn] add extractor

[shakeelmohamed]

Add support for LinkedIn.

Implementation is pretty straightforward, let me know if you need a more complete writeup.

Test cases pass locally:

$ python test/test_results.py linkedin

https://www.linkedin.com/feed/update/urn:li:activity:7381400030911500288/
# feed URL with video content - Found 1 media item
.
https://www.linkedin.com/posts/invalid-url-format
# invalid URL format - should trigger StopExtraction when post ID extraction fails
.
https://www.linkedin.com/posts/groveben_in-a-time-of-agentic-creation-taste-and-ugcPost-7403592769451335681-Jt4n
# posts URL with video content - Found 1 media item
.
https://www.linkedin.com/posts/the-brand-identity-group-ltd_since-2003-nuits-sonores-has-transformed-activity-7404006895822372867-2rvj?utm_source=share&utm_medium=member_desktop&rcm=ACoAAAkyN_sBOslkonzC5zCQEyrDt7AVDULHCNA
# posts URL with multiple photos and querystring - Found 4 media items
.
https://www.linkedin.com/posts/groveben_in-a-time-of-agentic-creation-taste-and-ugcPost-7403592769451335681-Jt4n?utm_source=test&utm_medium=test
# querystring removal verification - should work same as without querystring
.
----------------------------------------------------------------------
Ran 5 tests in 5.983s

OK