Generate SDKs for the untransformed source schemas

[leemhenson]

Related to #3367.

I want to be able to filter out certain fields in the source schema, but still be able to use that source's SDK to query that field. For example, I might write an additional typedef that matches the same type as the filtered field, and a custom resolver for it that queries the filtered field in the source, supplying the selection set from client query.

At the moment I can't do this, because filtering a field removes it from the SDK. I can work around it by renaming the field to "make space" in the stitched schema for the additional typedef, and then using a root-level filter to hide the renamed field. However, this makes the meshrc unnecessarily complicated, and means I have to refer to the renamed field on the SDK, rather than the original name. This also complicates thing unnecessarily.

Describe the solution you'd like

I can think of a couple of options:

a) Generate an SDK for the untransformed source schema, and place it in the context alongside the SDK that's currently being generated for the transformed source schema. e.g. something like

context.SpaceX.launch(id); // current, transformed SDK
context.SpaceXRaw.launch(id); // new, untransformed SDK

This is simple, but wouldn't allow the usage of any useful transforms like caches. We'd also have to think about name collisions.

b) Have the ability to define a source which is never actually stitched. This would allow the use of transforms, and the developer would be responsible for choosing the name.

My preference is for (b) and I will start building a PR for that, but I'm interested to hear about other opinions.