Allow for the easy creation of mocking requests

[erunion]

Discussed in #544

^{Originally posted by erunion May 21, 2020}
When working with an API, it's helpful to be able to mock out requests when unit testing client functionality. Since we've got the OpenAPI definition on hand for the API in question, we know everything about the API and should be able to accept and return mock data according to the contract.

Nock, the library that we'd likely use for supporting mocks, has an environmental flag that lets you completely disable Nock.

Along these lines, the two ideas that we have for mocking API requests out of SDKs are:

• Expose an API_MOCKS=true environmental variable that when enabled, causes the SDK to disable network traffic and mock out all requests.
• Alternatively, if you just want to mock out a single endpoint we should have a .mock() method that you could chain to your API call as such:

const sdk = require('api')('petstore.json');
sdk.mock().listPets().then(res => {
  // `res` is a 200 response and `res.json()` contains mock data based on the API
});

Unanswered questions right now are:

• How would we mock out failing requests? Would that entail you supplying .mock() with the status code you wish to cause .listPets() to return?