Second layer of encryption

[brickpop]

After #1 is ready, this PR adds the upper layer to handle targeted symmetric key encryption for multiple actors

• Encrypting metadata
• Encrypting actions
• Generating the list of encrypted symkeys
• The corresponding decryption
• E2E testing
• Workflows

[carlosgj94]

I think for now we can leave these three files here. But I agree that potentially this will become more of a generic feature and we might have to do it in a library/plugin of aragonette.

LGTM for now

[carlosgj94]

Good job on this helpers. They might look useless as most of them just call a function, but it's actually helpful the renamings.