Utils for discovering attached data and dropping in-memory data

[pzelasko]

This PR adds the following methods and properties:

• cut.iter_data() which iterates over (key, manifest) pairs of all data items attached to a given cut (e.g., ("recording", Recording(...)), ("custom_features", TemporalArray(...)))
• is_in_memory property for all manifest types to indicate if it contains data that is held in memory
• is_placeholder for non-cut manifests to indicate if a manifest is just a placeholder (has some metadata, but can't be used to load data)
• cut.drop_in_memory_data() which converts manifests with in-memory data to placeholders (this is useful for manifests that live longer than just dataloading to avoid blowing up CPU memory and/or slowing down the program)

[pzelasko]

Skipping the last test that fails due to numpy 2.0 release