support non-sha256 blobs in CAS

[cyphar]

Right now we outright reject blobs that aren't using sha256 as their blob algorithm. This is definitely not ideal, but was done as a stop-gap to avoid running into issues with "mixed hash" setups (we don't want to be pushing sha256 blobs into an image which is exclusively sha512 blobs).

This is effectively a carry of #197, but as mentioned in that thread we need to do more work than just removing the check.