Expose EffectType types enum

[challet]

From #1098 issue

• expose the EffectType enum so it can be relied on to test effect objects and infer their members

if (effect.type_i == Horizon.ServerApi.EffectType.trustline_created) {
    return effect.limit; // which doesn't exist on all effects
}

• Add a type_i to the Trade type, so it can also be tested and inferred. Including inferring it out from the previous example.

Note: I am not certain about from where it should be exported since it is a const from within a namespace only about types.

[Shaptic]

Seems good! Nice work 👏 It'll be exported at the top level of the Horizon module, I take it?

[challet]

Thanks ! Actually, no I've put it as Horizon.ServerApi.EffectType and that's what I wasn't sure about with

Note: I am not certain about from where it should be exported since it is a const from within a namespace only about types.

Should I move it to the top level then ?

[Shaptic]

Nope, in the Api is actually perfect 👍