Make FTD have its own the response

[ericholscher]

Every time we talk about addons performance, breaking up the API is the obvious next step. File tree diff is the one that makes the most sense to break out, so let's attempt that and see how it goes.

[humitos]

Just note that this will only affect PR builds, since FTD is not returned for regular versions.

[agjohnson]

We'll monitor the current addons API changes and can come back to this if we need.

[humitos]

I took a quick look at the "external" calls done by this function https://onenr.io/0VRVn8kMMRa and found https://onenr.io/0qwLDV3npw5 which says that it takes 20ms to perform a GET/HEAD.