Support non-Ed25519 endpoint IDs

Some projects would love to use iroh, but can't use Ed25519 keys for various reasons. To allow them to use iroh, we could extend the EndpointId to support other key types such as SecP256.

This creates a lot of changes. Some systems such as our current address resolution system as well as pkarr are fundamentally tied to Ed25519 keys, but these systems could just refuse to work with other keys.