Support execution environment signing

[mayaCostantini]

It could be convenient to be able to sign containers/execution environments from the command line using ansible-sign. The CLI would handle the signing part and interact with the automation hub container registry to store the signature and retrieve it from the image reference if necessary.

Different types of signatures could be generated, such as GPG and Sigstore/Cosign type signatures.
For Sigstore, the only barrier as of now is the lack of a Python client that supports container signing (this is not the case in sigstore-python currently), but we could try to implement this functionality. Some tools such as oras might be useful when it comes to interaction with an OCI registry, the rest of the signing logic being more straightforward and already handled by sigstore-python.