add toXDR and fromXDR for multiauth flow

[BlaineHeffron]

Addresses #976

Adds a toXDR and fromXDR for AssembledTransaction class. Also adds a txFromXDR to the Client class. Changes the swap test to use this flow.

Limitations: If you use the XDR multi-auth flow, you must resimulate before the final signAndSend call. This is due to the fact that we can't serialize the XDR of the transaction envelope with the results of the simulation. If we want to do this, we would need to add an AssembledTransaction type to XDR in Stellar Base.

[chadoh]

In the CHANGELOG entry that you add for this, let's be sure to mention that toJSON and fromJSON and txFromJson are now all deprecated, and to prefer the XDR variants.

If it were up to me, given the low-usage of this so far (literally probably only the CLI using this toJSON and fromJSON logic right now), I would just ship a breaking change. But we should probably wait until the next major version to do that. Maybe we can also add @deprecated markers to the JSDoc for these json-related methods.

[Shaptic]

If it were up to me, [...] I would just ship a breaking change. But we should probably wait until the next major version to do that.

I completely agree, but it's not the way we want to do things anymore (i.e. we want a single major version across the stack [up to the platform] to define protocol compatibility). Yet another reason why the SDK and the binding code should be able to evolve independently 😉

[chadoh]

Glad to have this, thanks!