`/artifacts/{kind}/{name}/{version}` should stream content

[iliana]

Right now both Nexus and Sled Agent read an entire update artifact into memory. They shouldn't.

(This might at some point track related issues in dropshot and progenitor.)

(From #717)

[iliana]

Sled Agent now streams: #1442

Nexus still needs this treatment.

[iliana]

This API has been deleted in #7431