Add a foxglove data platform source for experimental player

[defunctzombie]

The IterablePlayer uses a new mechanism for seekback to backfill the last message on each topic rather than the RandomAccessPlayer pattern of backfilling a specific amount of time. This new mechanism produces a better experience when seeking through data sources with infrequently published messages.

Research and implement a foxglove data platform IteratorSource (a thing the IterablePlayer uses as input). Beyond adding the new backfill capability for data platform users, this would allow us to re-use logic for playback and preloading across data sources by leveraging the single IterablePlayer rather than making a separate player to repeat preloading and playback logic.

[defunctzombie]

@foxymiles Assigning to you as the owner per our planning discussion.