standalone: want ability to run attestation server backed by real or mock RoT

[jordanhendricks]

Incremental work for #1067

This PR will cover running a VM attestation server via propolis-standalone. Via the standalone configuration TOML file, a standalone user can specify whether to use a real root of trust (via the "IPCC" backend, available on oxide hardware only) or use mock data generated by the vm-attest-proto crate.

The qualifying data for the instance will also be specified in the TOML file.