ContainerRegistry: Parse digest algorithm and value into separate fields

[euanh]

Motivation

SHA256 is the most common digest algorithm for blobs, but SHA512 can also be used and the image specification provides for other options. This change lays the foundations for generating and validating hashes other than SHA256, and reduces the use of untyped strings to represent digests.

Modifications

• Parse the digest into separate algorithm and value fields, instead of just validating
• Add support for parsing and generating SHA512 digests
• Throw a meaningful error instead of asserting when the registry returns an unexpected digest for an uploaded blob

Result

SHA512 digests can be parsed and verified

Test Plan

Existing tests continue to pass.
New parser tests added for SHA512 digests.