Add did:pkh for Aleo

[clehner]

• Register CAIP-2 namespace for Aleo: Aleo Blockchain Reference ChainAgnostic/CAIPs#84
• Add test vector and verification method type to did:pkh specification draft: Add support for Aleo w3c-ccg/did-pkh#15
• Add entry for Aleo in did:pkh specification
• Parse Bech32 account address
• Use BlockchainVerificationMethod2021 for verification method

This doesn't yet implement signing/verifying BlockchainSignature2021, only the construction of the DID document and verification method object that it will verify against. The verification method type may be changed to a different type if we find or develop a more specific one than BlockchainVerificationMethod2021/BlockchainSignature2021.
Edit: Signing/verifying is implemented in #360.

Parsing the constructed DID documents with ssi will depend on JSON-LD context for Blockchain Vocabulary (#347).

[bumblefudge]

Progress blocked by decision on whether to adapt existing signature suite or design a new one

[clehner]

Going to merge this even though the CAIP-2 namespace has not been confirmed as registered, and the did:pkh test vector PR has not yet been merged. If needed this can be changed later. Verification method / signature suite implementation using Aleo keypairs is still open at #360.