print image digest from wkg oci push

[yoshuawuyts]

Closes #126 by enabling wkg oci push to print the digest after pushing. This enables us to make progress towards bytecodealliance/wkg-github-action#6, which is needed for integrating with e.g. actions/attest. Thanks!

Example Output

❯ cargo run --bin wkg oci push ghcr.io/yoshuawuyts/wasi/filesystem:0.2.0 ./webassembly_wasi_filesystem.wasm
pushed: ghcr.io/yoshuawuyts/wasi/filesystem:0.2.0
digest: sha256:1ef9132612ec97eb5bca4fed5a5177e8cb0c5ebabdb4cfa7e04a295386c05e4e

[thomastaylor312]

Thanks @yoshuawuyts!