[DA-253] storage api methods with asset service and local filesystem backends

[aletts]

This adds an abstract StorageBackend class with AssetServiceBackend and LocalFileBackend. One backend can be chosen as the primary for reads, while any number of backends can be chosen for writes.

This initial API is modeled to support the existing asset service functionality for projects and to prioritize similarity between files saved locally and the asset service (to serve as a mirror). As such, the file organization is flat and uses uuid's. Future plans may involve a more convenient structure - and may leverage a suitable existing library.

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