Encode public key in dids (rather than nillion1 address)

[tim-hm]

Currently dids follow the format: did:nil:mainnet|testnet:nillion1_rest_of_address. This approach requires the receiver to hold the associated public key to validate the signature (this is because the nillion address is a hash of the public address).

There are advantages to removing this state. For example, it makes it easier to have stateless requests to some services in the nillion network, allows an account that doesn't exist in nilDB, with delegate permissions, to run a query or upload data, and it would make it easier to support requests from other chains.

Additionally, the NUC spec is adopting this format for similar reasons.

So, we should migrate NilDB to use dids shaped as did:nil:public_key_as_hex.

[tim-hm]

@pablojhl as this relates to the NUC and NilVM signature work i've assigned it to you.