feat: add support for async based resource operation handlers

[pablopalacios]

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

This PR enables fetchr to be used without any callback.

Before

function create(req, resource, params, body, config, callback) { 
    callback(err, data, meta)
}

After

async function create({ req, resource, params, body, config }) {
    return { data, meta }
 }

It's still possible to use the old signature since we detect the new one based on the number or arguments defined in the handler.

This change also make it simple to create handlers that do not require any argument:

Before

function create(req, resource, params, body, config, callback) {
    callback({ ok: true });
}

After

async function create() {
    return { data: { ok: true } };
}

[pablopalacios]

@redonkulus yes, I tried with npm link on our main website code with both versions running (callback and async). All good!