add fetchOptions to IRequestOptions to replace fetch, httpMethod, credentials, etc

[tomwayson]

We should replace the individual IRequestOptions like httpMethod and credentials that control the fetch() init object with a single parameter like fetchOptions or fetchInit that will be passed directly to fetch. This should resolve issues like #810 and also allow us to remove passing in a custom implementation of fetch() via the fetch option.

Removing the existing parameters would be a breaking change. We could in theory add the new option first and deprecate the old ones until we later remove them as part of a larger major bump.