The motivation of this change is to move away from tying signing to be
specifically for ed25519. The reason being that the protocol will want move
towards two different kinds of signing – the node signing artifacts, and an
author signing artifacts.
This change captures the former, node signing, by introducing a `Device` type
that is the `NodeId` – a PublicKey underneath the hood – and a signing
mechanism. This allows the replacement of the `Signer` trait being used – which
always assumed a `PublicKey`. Instead, the `Device` is constructed with
`NodeId`.
In `radicle-cob`, a signer is expected to implement
`signature::Signer<ExtendedSignature>`, and everywhere in `radicle`,
`radicle-node`, `radicle-cli`, and `radicle-remote-helper` is expected the
signer is expected to implement `signature::Signer<Signature>`.
A `Device` implements both of these but only requires
`signature::Signer<Signature>` to do so – since an `ExtendedSignature` is
essentially `(PublicKey, Signature)`, and the `NodeId` of the `Device` can be
used.
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