Re-use resource class for remote sideloads (avoid memory leak)

[PChambino]

Dynamically creating resource classes for each sideload is expensive on allocations and it was causing a memory leak on carwow's production applications since these Class objects were not being freed.

Not sure if there was anything in particular to our applications that was preventing these Class objects from being freed, but even if there is no memory leak on a simple application, it is still expensive and slow in terms of allocations.

When running a memory-profiler and doing an HTTP requests that includes/sideloads resources from a remote resource, you should be able to see something like:

retained memory by location
-----------------------------------
   2.23 kB  /usr/local/bundle/gems/graphiti-1.3.6/lib/graphiti/query.rb:81

This PR proposes defining a static resource class for remote sideloads which can be re-used: RemoteSideloadResource.