feat: add dynamic item source

[jasongdove]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces support for dynamic playout items, allowing PlayoutItem JSON documents to be fetched over HTTP(S) at playback time. It adds reqwest as a dependency, updates the playout schema, and implements resolution logic within ChannelSession to handle template expansion, request headers, and recursion prevention. Feedback on the changes points out critical compilation errors in resolve_dynamic_item where the code attempts to move non-Copy fields out of shared references (specifically within the tracks and watermark checks); using references instead of direct moves is recommended to resolve these errors.