Horizon v1.0.0 Compatibility

[overcat]

#261

• feat: add support for /offers end-point with query parameters.
• feat: add support for /accounts end-point with ?signer and ?asset filters.

[ire-and-curses]

Tamir is out for a week now. @leighmcculloch do you think you could take a look at this?

[leighmcculloch]

@ire-and-curses Apologies this fell through my notifications. I don't have much context on what it means to be Horizon v1 compatible and my queue is pretty deep right now so I'm unlikely to get to this PR before @tamirms returns. Let me know if there is particular urgency here I am not aware of.

[overcat]

It is worth mentioning that in horizon 1.0, the offer_id in the offers endpoint is a string type, but in Java code we will parse it into a Long type. Do you think we need to change it? I read the horizon code and it is still of int64 at the bottom, so I don't think we need to modify it.

Let's put all type-related changes in another PR.

[tamirms]

@overcat this looks great! we should keep offerId as a long as you've noted