feat: add support for #EXT-X-PART

[brandonocasey]

This pull request adds support for the #EXT-X-PART tag and all of the attributes currently in the specification for it.

See:

• https://developer.apple.com/documentation/http_live_streaming/enabling_low-latency_hls#3282436
• https://tools.ietf.org/html/draft-pantos-hls-rfc8216bis-08#section-4.4.4.9