Add resource middleware to non batch resources

[magicmark]

Oops! We forgot to apply custom middleware to non batch resources.

What's going on here?

dataloader-codegen works by having the user define a config file of "resources" [1] that they want to generate dataloaders for.

[1] A "resource" = a function that returns data - e.g. a Promise from a call to fetch. In Yelpy terms, this is an API function exported by a node clientlib

A config file might look like this:

dataloader-codegen/examples/swapi/swapi.dataloader-config.yaml

Lines 9 to 14 in 5aa5db6

	resources:
	getPlanets:
	docsLink: https://swapi.co/documentation#planets
	isBatchResource: true
	batchKey: planet_ids
	newKey: planet_id

You can see that we declared getPlanets to be a batch resource, by passing isBatchResource: true.

dataloader-codegen has two seperate codegen methods - one for batch resources, and one for non batch resources:

• dataloader-codegen/src/implementation.ts

  Line 27 in 5aa5db6

  function getBatchLoader(resourceConfig: BatchResourceConfig, resourcePath: ReadonlyArray<string>) {

• dataloader-codegen/src/implementation.ts

  Line 385 in 5aa5db6

  function getNonBatchLoader(resourceConfig: NonBatchResourceConfig, resourcePath: ReadonlyArray<string>) {

What's "middleware"?

Functions that run before and after the call to the underlying resource - allowing the user to globally transform the arguments/response before passing it back to userland.

Our generated code should wrap the call to the underlying resource with middleware, rather than calling the underlying resource directly.

See: https://github.com/Yelp/dataloader-codegen/blob/master/API_DOCS.md#getloaders-arguments

What's the problem?

The codegen for batch resources and non-batch resources are separate methods (see above) and share no common code. When we added support for middleware to this library, we never added it to the getNonBatchLoader codegen 😬

What does this PR do?

Fixes the above issue, by pulling the middleware wrapped call to the underlying resource out into a seperate codegen method, to be called by both getNonBatchLoader and getBatchLoader.

[mxmul]

Maybe this should this be a regular throw? It can be a little confusing to mix Promises and async/await in the same function.

[magicmark]

ya we can do that (should be functionally equivalent)