Support Bundle API tweaks for range requests, error codes

[smklein]

• Support bundles would like to be able to propagate streaming bodies which also include error codes in the API spec. As an example of this, see: Support bundle download endpoints could distinguish success and failure status codes in OpenAPI #7346 . This is currently limited by the fact that FreeformBody drops response headers, which would be necessary for range requests.
• Support bundles would like to implement range requests in their API, by propagating requests from Nexus to Sled Agent. Unfortunately, although Nexus can parse these headers, they cannot be sent with the progenitor API as-is. This requires Add .build() to allow manipulation of reqwest::RequestBuilder prior to typed send() progenitor#750 to be able to propagate request headers.