HTTPRequest execution should return a Promise instead of a Publisher

[marcantoinefortier]

Current interface

interface HttpRequest {
    @JsName("execute")
    fun execute(cancellableManager: CancellableManager): Publisher<HttpResponse>
} //                                                     ^^^^^^^^^

Desired interface

interface HttpRequest {
    @JsName("execute")
    fun execute(cancellableManager: CancellableManager): Promise<HttpResponse>
} //                                                     ^^^^^^^

Why?

A Promise has a stronger contract than a Publisher regarding the possible outcome of the HTTP request. The current interface is a bit "misleading" in the sense that a Publisher can emit multiple values in time, while a Promise would explicitly enforce that you will receive only one value/error.

Also, returning a Promise would allow multiple HTTP requests to be chained more easily.