Improve player item tracker API and stream type transition support

[defagos]

Description

This PR brings two improvements:

• Custom item trackers are now easier implement thanks to an extended API contract which avoids the need for explicit publisher subscriptions (Improve tracker API contract #560).
• Stream types used by trackers are not extracted from metadata anymore but rather from the stream itself, which allows supporting live to on-demand transitions at the end of sports livestreams e.g. (Revisit Commanders Act implementation to support stream type transitions #607).

Changes made

• Update PlayerItemTracker protocol.
• Update comScore and Commanders Act implementations to match the improved protocol.
• Update comScore and Commanders Act implementations to remove the stream type inputs.
• Update tests accordingly.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• The playground has been updated (if relevant).