feat: support web3name and linked accounts in the identity proof

[ntn-x2]

Fixes https://github.com/KILTprotocol/ticket/issues/2668.

Main changes

• The ProofLeaf enum has been restructured to now contain three possible types of leaves: DidKey, Web3Name, and LinkedAccount
• The template dip-producer now also has the lookup and the web3name pallets deployed, to showcase the new proof features
• The Identity generic type of the IdentityProvider trait has been renamed to Success and promoted to associated type.

pub trait IdentityProvider<Identifier, Identity, Details = ()> {
    type Error;

    fn retrieve(identifier: &Identifier) -> Result<Option<(Identity, Details)>, Self::Error>;
}

becomes

pub trait IdentityProvider<Identifier> {
    type Error;
    type Success;

    fn retrieve(identifier: &Identifier) -> Result<Option<Self::Success>, Self::Error>;
}

• the DipProvider runtime API now takes a generic ProofRequest as input, which can be specified by each runtime, The KILT one includes the DID identifier of the subject, the keys and accounts to reveal, and whether the proof should include the web3name, if the DID subject controls one.