Update NFT DID to be purely generative.

[oed]

This PR simplifies the NFT DID specification by a lot. This is achieved by making it purely generative. In order to use NFT DID going forward an object-capability in the form of a ChainProof would be needed to be used. This greatly reduces the complexity of the NFT DID overall since a Ceramic node would no longer need to rely on an external Blockchain indexer.

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[oed]

@stbrody Given a ChainProof you would only need a blockchain light-client to verify its validity. I think NFT DID is useful beyond Ceramic, but in the case of Ceramic I could see us supporting Ethereum and all Eth L2s potentially.

[oed]

Note to self: Would be useful to be able to put arbitrary delegations as part of the tokenURL metadata. This would only work if the metadata is generated from on-chain (for example, like uniswap LP position NFTs). We could add a new field to the metadata object which could include a DID uri or an IPFS hash containing a capability object.

This would allow custom NFTs to be created where the owner can change delegations as they see fit. In particular this would be useful if a DAO want's to vote on who they delegate access to.

cc @ukstv you might find this interesting.