How to build Claimable Balance Predicate on txbuild?

[abuiles]

We should offer some high level helpers in txnbuild to make it easy for people to create claimable balances predicates.

Following the spirit of txnbuild users, final users should need access to the XDR library.

We could have some helpers like:

txnbuild.AndPredicate(left, right)
txnbuild.OrPredicate(left, right)
txnbuild.NotPredicate(left, right)
txnbuild.AbsBeforePredicate(int64)
txnbuild.RelBeforePredicate(int64)

[abuiles]

@Shaptic I added some initial helper here to help us write the integration tests abuiles@6d3d8e8 -- we can use this for now internally, while we define the final interface for txnbuild.

[abuiles]

@2opremio @tamirms can you check the interface here abuiles@6d3d8e8#diff-53889a0fa496ed0e06224eaebe4319d7R25 -- this is just an initial version to help us write integration tests.

[Shaptic]

I cherry-picked the above commit as part of my predicate-tests branch (see exact file), and added some things as I needed them for the tests.

[abuiles]

We had a conversation and decided to move forward with the factory methods to create xdr.ClaimPredicateand keep a txnbuild struct to wrap the destination as a string and then the predicate as an xdr struct.

We'll have the following in txnbuild

type Claimant struct {
	Destination string
	Predicate   xdr.ClaimPredicate
}

func (c *Claimant) BuildXDR() (xdr.Claimant, error) {
  ...
}
func (c *Claimant) FromXDR(xdrClaimant xdr.Claimant) error {
  ...
}

And then people can create new claimants combining the struct above and the factory methods

txnbuild.Claimant{
  Destination: "GGGG",
  Predicate:  txnbuild.AbsBeforePredicate(200000000)
}

[Shaptic]

This was implemented & merged in the above.