External Content IDs for blockchain: Adopt chain agnostic specifications

[friedger]

In the current version of NIP-073, content IDs for blockchain addresses and blockchain txs are described similarly to the chain agnostic project. In particular, blockchain addresses are specified as blockchain accounts at https://chainagnostic.org/CAIPs/caip-10

The main difference is the blockchain specifier. CAIP-010 defines chain_id that consists of a blockchain namespace and a reference. NIP-073
uses a language like "any layer 1 blockchain [a well known name] plus chain ID [a number]".

Suggestion

• Replace the specification of "Blockchain Address" with the one of CAIP-010
• Replace the specification of "Blockchain Transaction" with the one of new CAIP - Transaction Object Addressing ChainAgnostic/CAIPs#221
• Add human-readable short names as an alias for chain_ids, like ethereum for eip155:1, bitcoin for bip122:1