list_objects: check S3 API & fjall part implementation

[iwanbk]

current list_objects and list_objects_v2 implementation was a naive translation of the Sled based implementation.

• add more tests, especially the usage of prefix as continuation_token
• See if we could improve it using fjall way. For example using prefix directly https://docs.rs/fjall/latest/fjall/struct.PartitionHandle.html#method.prefix