Represent blockchain_account_id part of VerificationMethodProperties according to EIP-55 #13

Is your feature request related to a problem?

The did:ethr specification indicates that all addresses are case-insensitive, but that blockchain_account_id may be represented according to EIP-55. It may be advantageous to do so:

https://github.com/decentralized-identity/ethr-did-resolver/blob/master/doc/did-method-spec.md#method-specific-identifier

EIP-55 describes a way of checksummed public keys: https://eips.ethereum.org/EIPS/eip-55

This representation is only relevant to the outputted DID Document, but does not have to be represented as such internally.

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