Refactor http-client

[mistermoe]

Currently http-client methods returns DataResponse or ErrorResponse, both of which are HttpResponses. The Developer Experience is a bit awkward because

1. returning status and headers doesn't really provide any benefit
2. it's not entirely evident how callers should figure out whether the response received includes an error

We may want to consider something a bit more high level that:

1. drops inclusion of headers and status in what's returned
2. throw errors instead of returning them

for example, getOfferings would just return Offering[] on success or throw an error otherwise.

[kirahsapong]

Introduced msw for mocking requests in http-client tests in #113. Currently only configured for node, will need to configure for browser tests in a followup PR.

[kirahsapong]

Closed by #113