standalone: add simple instance attestation server

[jordanhendricks]

addresses #1068

TODO:

• clean up how we get the attestation port
• test on a gimlet
• test with more up-to-date vm-attest-proto bits in the guest image

Example TOML Configurations

"Mock" RoT (mock data)

[attestation]
backend = "mock"
port = 3000
pki_path = "/home/jordan/src/vm-attest-proto/target/debug/build/vm-attest-trait-19b74ae95e889c80/out/test-alias.certlist.pem"
log_path = "/home/jordan/src/vm-attest-proto/target/debug/build/vm-attest-trait-19b74ae95e889c80/out/log.bin"
alias_key_path = "/home/jordan/src/vm-attest-proto/target/debug/build/vm-attest-trait-19b74ae95e889c80/out/test-alias.key.pem"
instance_uuid = "db5bf54c-48c5-4455-a1e1-6c7dfc26e351"
boot_digest = "be4df4e085175f3de0c8ac4837e1c2c9a34e8983209dac6b549e94154f7cdd9c"

Real RoT

# IPCC backend (communicates with real RoT)
[attestation]
backend = "ipcc"
port = 3000
instance_uuid = "db5bf54c-48c5-4455-a1e1-6c7dfc26e351"
boot_digest = "be4df4e085175f3de0c8ac4837e1c2c9a34e8983209dac6b549e94154f7cdd9c"

[flihp]

I've added to this to get it working w/ the latest vm-attest. Branch is here: https://github.com/flihp/propolis/tree/standalone-new-vm-attest. With these changes I've tested both the IPCC & mock backends successfully using the image at /staff/flihp/vm-instance.raw.gz

[flihp]

Not a problem w/ this code, but the VmInstanceRotMock type has a confusing name. The type it wraps may be a mock impl, but VmInstanceRotMock itself isn't. I'll try to come up w/ a better name to make this less inaccurate.

[flihp]

This type shouldn't be so hard to construct. We should probably include the digest identifier (sha-256 in this case) in the string from the config, and provide a from_something constructor for the Measurement type.

[flihp]

PR adding a FromStr impl to Measurement type is here: oxidecomputer/vm-attest#49. A commit that uses this to parse the boot_digest from a propolis-standalone TOML config is here: flihp@6fe8547. This commit was tested on atrium. NOTE: this requires taht the boot_digest field in the TOML be updated to include the IANA NIH hash algorithm prefix string like: sha-256;{hex}