select base device report conformance via content negotiation

[karenetheridge]

As we change device report formats, we can select the base schema conformance via:
Content-Type: application/schema-instance+json; schema="https://conch.joyent.us/json_schema/request/base_device_report_schema_v1.2"

[karenetheridge]

Blocked on #943, and also on the JSON Schema specification figuring out what to do here (draft 2019-09 says to do what I wrote above, but https://www.w3.org/TR/dx-prof-conneg/ seems to be modifying the syntax a bit).

[karenetheridge]

We can also use this to switch between different report formats and validation types, e.g. "preflight from livesys" vs. "production postflight from serengeti".