[nexus] Support Bundle HTTP endpoint implementations

[smklein]

PR 5 / ???

Implements support bundle APIs for accessing storage bundles.

Builds atop the API skeleton in:

• [nexus] Support Bundle API Skeleton #7008

Uses the support bundle datastore interfaces in:

• [nexus] Support Bundles Datastore methods #7021

Relies on the background task in:

• [nexus] Support Bundle background task #7063

TODO before merging:

• Integration tests?
• If possible, make range requests work correctly?

[wfchandler]

Thanks @smklein, I have no complaints with this one. I'm not very familiar with Progenitor, though, so take my approval with a grain of salt.

[smklein]

Thanks @smklein, I have no complaints with this one. I'm not very familiar with Progenitor, though, so take my approval with a grain of salt.

I've been discussing some follow-ups with @ahl in both:

• Support bundle download endpoints could distinguish success and failure status codes in OpenAPI #7346 (to propagate the streaming response bodies, while also ideally plumbing out response headers with error codes in openapi)
• and Add .build() to allow manipulation of reqwest::RequestBuilder prior to typed send() progenitor#750 (to be able to propagate headers through the progenitor client interface, talking to sled agents)

I think the resolution here will require progenitor and dropshot changes to be done - I can file some issues related to this usage for support bundles before merging.

[smklein]

Filed #7356