wire up omaha::Hash<T> with DynDigest

[dongsupark]

Currently hash_on_disk only supports only Sha256, it simply relies on Sha256, although omaha::Hash has both Sha256 and Sha1.

For potential supports of Sha1, it might be interesting to look into digest::DynDigest of Rustcrypto that supports multiple hash functions at the same with generics.

[pothos]

It would be good to support sha1 hashes in the Omaha response and check them if available. With an old Nebraska version one only get sha1 when syncing from the public Nebraska.

[pothos]

Change done in #47