Add sled-agent API to report multirack join service status - #11535

control: job assigned to worker 01M5MND6403XQZGG6BW6AVEKYK [factory aws, i-0576977c746cfb69e] (queued for 24 s)
...
task: starting task 0: "setup"
task: process exited: duration 18472 ms, exit code 0
...
task: starting task 1: "rust-toolchain"
task: process exited: duration 8508 ms, exit code 0
...
task: starting task 2: "authentication"
task: process exited: duration 6 ms, exit code 0
...
task: starting task 3: "clone repository"
task: process exited: duration 6169 ms, exit code 0
...
task: starting task 4: "build"
...
| + [[ linux == \i\l\l\u\m\o\s ]]
| + rm -rf /var/tmp/omicron_tmp/crdb-base-build
| + unset TMPDIR
| + echo 'files in /var/tmp/omicron_tmp (none expected on success):'
| files in /var/tmp/omicron_tmp (none expected on success):
| + find /var/tmp/omicron_tmp -ls
|    258225     24 drwxr-xr-x   2 build    build       20480 Oct 23 15:47 /var/tmp/omicron_t [...]
| + rmdir /var/tmp/omicron_tmp
task: process exited: duration 1744149 ms, exit code 0
|W| found 5 output files
|W| uploading: /work/cargo-build-analysis.jsonl (1816812 bytes)
|W| uploaded: /work/cargo-build-analysis.jsonl
|W| uploading: /work/environment.json (108 bytes)
|W| uploaded: /work/environment.json
|W| uploading: /work/oxidecomputer/omicron/target/nextest/ci/junit.xml (704604 bytes)
|W| uploaded: /work/oxidecomputer/omicron/target/nextest/ci/junit.xml
|W| uploading: /tmp/nextest-run-archive.zip (3266117 bytes)
|W| uploaded: /tmp/nextest-run-archive.zip
|W| uploading: /tmp/nextest-chrome-trace.json (4427043 bytes)
|W| uploaded: /tmp/nextest-chrome-trace.json