Allow blockchainAccountId

[clehner]

blockchainAccountId is like a generalization of ethereumAddress using CAIP-10. Blockchain account IDs representable with CAIP-10 and using Secp256k1, besides Ethereum, include Bitcoin, Dogecoin, Cosmos, and Tezos.

This PR adds blockchainAccountId as an allowed verification method property in the readme/docs, code, and example DID document.

More info:

• https://www.w3.org/TR/did-spec-registries/#blockchainaccountid
• https://w3c-ccg.github.io/security-vocab/#blockchainAccountId
• https://github.com/ChainAgnostic/CAIPs/blob/master/CAIPs/caip-10.md

Some other implementations of blockchainAccountId:

• https://github.com/decentralized-identity/did-jwt/blob/73cba89d6a0cbbff277ad0d5f4c5dd258de1d773/src/blockchains/index.ts
• https://github.com/spruceid/ssi/blob/15e944620e20b31b4644edad094e01ff7b418e44/src/caip10.rs#L44-L79