fix(ref-imp): allow setting controller property

[clehner]

Allow setting the controller property of a public key entry (verification method object), as described here: https://identity.foundation/sidetree/spec/v1.0.0/#add-public-keys

[csuwildcat]

@thehenrytsai this would be a breaking consensus change for existing implementations. Can we accept this and plan an ION release separately?

[zkwzk]

hello, any update for this change? does sidetree plan to support controller property

[thehenrytsai]

Was waiting for the contributor to update the PR to merge this. I'd love to know more about the scenarios/use cases of this property from anyone who's an advocate for the feature, I can probably make the requested changes myself and merge it in!