Plugin interface for identity to recipient conversion?

Although key generation is too plugin-specific to handle with a unified interface, and is best left to a direct invocation of the plugin, converting the identity string to a recipient string could be abstracted away.

If we add this plugin protocol extension, we should expose it in the CLI from age-keygen -y and in the API from plugin.Identity.Recipient, making plugin.Identity.Recipient().String() actually return the recipient encoding if supported.