createClaimableBalance: create claimant predicate from JSON object

[quietbits]

Currently, all claimant predicate creation helpers are functions in createClaimableBalance, which makes it harder to dynamically compose them. It would be great if there was an option to pass a JSON object instead.

[abuiles]

Here's a reference implementation in go https://github.com/stellar/go/blob/master/xdr/json.go#L84 -- this is actually how predicates are shown in Horizon