Add helpers to create CAP23 operations.

[abuiles]

Extend the Operation class with two new helpers which allow the creation of CAP23 related operations:

• Add Operation.createClaimableBalance.
  Extend the operation class with a new helper to create claimable balance operations.

const asset = new Asset(
  'USD',
  'GDGU5OAPHNPU5UCLE5RDJHG7PXZFQYWKCFOEXSXNMR6KRQRI5T6XXCD7'
);
const amount = '100.0000000';
const claimants = [
  new Claimant(
    'GCEZWKCA5VLDNRLN3RPRJMRZOX3Z6G5CHCGSNFHEYVXM3XOJMDS674JZ',
     Claimant.predicateBeforeAbsoluteTime("4102444800000")
  )
];

const op = Operation.createClaimableBalance({
  asset,
  amount,
  claimants
});

• Add Operation.claimClaimableBalance.
  Extend the operation class with a new helper to create claim claimable balance operations. It receives the balanceId as exposed by Horizon in the /claimable_balances end-point.

const op = Operation.createClaimableBalance({
  balanceId: '00000000da0d57da7d4850e7fc10d2a9d0ebc731f7afb40574c03395b17d49149b91f5be',
});

[Shaptic]

Again, I'm not a JS expert by any means, but everything LGTM 👌

[Shaptic]

Wait, why length < 2?

Also, the docstring above says opts.claimableBalanceId which doesn't match opts.balanceId here.

[abuiles]

good catch! Actually we don't need the length check, we can delegate that to the XDR library.

[Shaptic]

Since we decided not to compress the XDR, this should be updated with the leading 0s!

[Shaptic]

Doesn't the latter condition fulfill the former condition?
That is, if opts.claimants = [], then I think !opts.claimants will be true.
Maybe a type check for Array makes sense here, instead?

[abuiles]

good point, updated to. (!Array.isArray(opts.claimants) || opts.claimants.length === 0)