Proposal: Add new attributes for custom key algorithms

[druide]

I propose to add new attributes of Multikey:

AlgorithmName (0x0c) : The key algorithm name of a custom key codec.

KeyType (0x0d) : The key type of the custom codec (0 or not set - public, 1 - secret).

In my use case, I use experimental algorithms which are not included to the multicodec table yet, so need a way to identify them. In the case of a custom key algorithm, the <key codec sigil> is set to 0, and the AlgorithmName is set to the (well-known) name of the algorithm. Examples: "Falcon-512", "FAEST-128f".
Also, need a way to distinguish between secret and public keys of the algorithm, for which purpose is the KeyType attribute.

Also, new attributes of Multisig:

AlgorithmName (0x07) : The signature algorithm name of a custom signature codec.

Nonce (0x08) : Nonce bytes, for codecs without signature randomization.

PublicKey (0x09) : Public key raw bytes.

In the case of a custom signature algorithm, the <signing codec sigil> is set to 0, and the AlgorithmName is set to the (well-known) name of the algorithm.
In my use case, nodes are signing some external data. By best practises of security, a "Nonce" should be used to protect keys, but not all of the signature algorithms have it built-in. As the node cannot update provided data with a nonce, it should protect its own signature by using own "Nonce", what is the purpose of having this field as an attribute of the signature. The semantics of the "Nonce" is to append it to the data before signing and verifying.
The "PublicKey" attribute purpose is to identify the signer and to verify the signature, in the case when data doesn't have a public key included.

Regards!