Start time API improvements

[defagos]

As a developer integrating Pillarbox I want to be able to handle start times in a way similar to other platforms, that means:

• Define a temporary start index and start time to resume playback at.
• Define a temporary start time for all items in a playlist.
• Define an intrinsic start time always always applied to an item of a playlist.

We should also support a start date if possible.

Hints

• The Apple implementation does not handle scenarios where each item in a playlist must have a different time to resume once from.
• Maybe we could add a PlayerItem.resume(_:) API that makes it possible to define a temporary time, consumed once applied (and overridden otherwise).
• This same API could then possibly used to also implement the Player.resume(_:in:) API as well (the remaining implementation being finding the index of the item to resume at).
• Intrinsic start times associated with player items are already supported (PlaybackConfiguration).

Acceptance criteria

• The three above scenarios are supported with an ergonomic API.
• If a temporary start time and an intrinsic start time are defined, the temporary start time wins (only once).
• Start date APIs are provided if possible.

Tasks

• ...