Expose fetch headers and options

[nicholasio]

Is your enhancement related to a problem? Please describe. Currently, there's no way to pass additional fetch header or options to the fetch request.

Describe the solution you'd like

Provide an API to allow consumers to pass additional headers or options to fetch. This would most likely be by passing an additional object to the provider or extracting the creation of the HTTP client and passing a client option to the provider.

Designs

Describe alternatives you've considered

Additional context