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.

Start with the plugin protocol extension and the existing age-keygen -y and plugin.Identity.Recipient entry points. Define how identity-to-recipient conversion is represented, then verify that supported plugins return the recipient encoding through both the CLI and API, including plugin.Identity.Recipient().String().

Written by the indexing model from the issue text.