OperationCallBuilder should include a method to provide optional join arguments to Horizon.

[marcinx]

Is your feature request related to a problem? Please describe.
Recently Horizon started supporting an optional join parameter on its /operations endpoint. Unfortunately it currently isn't possible to pass the join argument to the OperationCallBuilder (https://stellar.github.io/js-stellar-sdk/OperationCallBuilder.html) -- at least I am not aware of a way to do so.

Describe the solution you'd like
OperationCallBuilder should include a method .join() that propagates to the join parameter to the /operations endpoint.

Describe alternatives you've considered
Currently we need to query operations and then query the transaction for every single result to obtain the memo. Using the join parameter lets us query operation details and memo in a single round-trip.

Additional context
The join method should allow for comma separated values, to allow flexibility in the future, when e.g. effects are added to the list of supported join options.

@abuiles @tomquisel

[ire-and-curses]

For reference, Go SDK implementation is here: stellar/go#1521

[abuiles]

@marcinx this has been released in 3.0.1 https://github.com/stellar/js-stellar-sdk/releases/tag/v3.0.1