Add `sha512` support to .NET SDK Container Builds

### Is your feature request related to a problem?

Three digest algorithms are defined in the OCI spec: `sha256`, `sha512`, and `blake3`.
Currently, .NET SDK container builds only support images/layer manifests that use `sha256`.

### Describe the solution you'd like

`sha512` support is spotty at the moment. For example, Azure Container Registry doesn't support uploading manifests that use `sha512` digests. Eventually, however, I expect that `sha512` will be supported more broadly among tooling and registries. .NET should not be a blocker here.

### Alternatives you've considered

_No response_

### Additional context

Related: `blake3`, also part of the OCI spec, is blocked on https://github.com/dotnet/runtime/issues/80900.

Start in the .NET SDK container-build implementation and trace where OCI image and layer manifests currently assume sha256. Compare the digest handling with the OCI specification, then verify that container builds can support sha512 manifests without blocking existing sha256 behavior.

Written by the indexing model from the issue text.