[API] Client can request signed locators for specific blobs from API server
For uploading or future "synchronize" operations where some data may already be present on the remote Keep instance, it would be very useful if the client can ask the API server directly for signed locators from a list of blobs. The server will check if the blobs exist and are readable by the user and then returns signed locators suitable for submitting a manifest.
This depends on the client already having permission to read the blobs in question, which means they are already referenced by a collection. Currently we expect clients to know the relevant collection UUID in any situation where it's appropriate to read a blob -- in that case the client can already retrieve fresh signatures via
collections.get. If a future situation makes this particularly difficult for a client, this feature would simplify client behavior at the cost of maintaining a cross-reference of blob→collection in the API server's database.