Consider removing the endpoint specific request builder abstraction

[oguzkocer]

Such as UsersRequestBuilder, PluginsRequestBuilder etc. As discussed here, it's not clear if having these abstractions is helpful. We'd like to implement a few more endpoints and revisit this decision.

I've added this to 0.2 milestone for now, so that we don't completely forget about it. If we don't want to decide in 0.2 milestone, we can push it to 0.3, but I don't think it's a good idea to use the Future milestone for this.

[oguzkocer]

We are now generating these abstractions from #[derive(WpDerivedRequest)]. All of the current abstractions have a clear purpose:

• Endpoint types allow us to use them on their own, and more importantly, write unit tests for them.
• RequestBuilder types allow manual request building and execution. They are also used by async request executors and planned to be used by blocking request executors.
• AsyncRequestExecutors represent our public API.