A CAS storage provider that stores files as blobs on disk.
Return True if the blob with the given instance/digest exists.
Return a file-like object containing the blob.
If the blob isn’t present in storage, return None.
Delete the blob from storage if it’s present.
Return a file-like object to which a blob’s contents could be written.
Commit the write operation. write_session must be an object returned by begin_write.
The storage object is not responsible for verifying that the data written to the write_session actually matches the digest. The caller must do that.