feat(client-factory): adding client factory

[billxinli]

Sometimes we may need to override the client function based on the spec, environment or special circumstances, such as test scenarios in test environment, where a mocked out fetch is required.

The constructor takes a fetch argument, however this can not be easily customized during run time, and the fetch function is called with the standard arguments, and it does not have any awareness to the spec that is associated to the call.

const api = new OASRequest({
  server: 'https://httpbin.org:443',

  clientFactory: function (url, options, spec) {
    const { operation: { operationId } } = spec

    if (process.env.NODE_ENV === 'test') {
      if (operationId === 'httpGet') {
        // Return a mocked out Response object, which the caller client can use with all standard fetch response interfaces
        return () => new Response(
           JSON.stringify(getExampleFromSpec(spec, process.env.TEST_SCENARIO)), 
           { 
              status: getStatusFromSpec(spec, process.env.TEST_SCENARIO) 
           }
        )
      }
    }

    return fetch
  }
})