fix ls-apis breakage

[davepacheco]

This PR:

• Fixes the problem described in ls-apis broken due to the new clickhouse admin APIs #7121.
• Adds a new check that the tool found a producer for every API in the metadata. (I had to add an exception for "dsc" because it's not deployed. But the exception is driven by metadata, not hardcoded.) This ensures that we catch similar problems in CI in the future.

These are separated into two commits if you want to see which part is which. They're in the reverse order -- I added the test first to make sure it would fail without the fix.

Fixes #7121.

[davepacheco]

With just the check in place but no fix, the command fails as expected:

dap@ivanova omicron-work $ git reset --hard d7a02268836aaf58ef2fa37be1c715eb1647cf10
HEAD is now at d7a022688 add check that all APIs are produced at least once
dap@ivanova omicron-work $ cargo xtask ls-apis apis
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.53s
     Running `target/debug/xtask ls-apis apis`
   Compiling omicron-ls-apis v0.1.0 (/home/dap/omicron-work/dev-tools/ls-apis)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.10s
     Running `target/debug/ls-apis apis`
loading metadata for workspace omicron from current workspace
loading metadata for workspace propolis from /home/dap/.cargo/git/checkouts/propolis-12517f89d3d9f483/8f8fbb7/Cargo.toml
loading metadata for workspace maghemite from /home/dap/.cargo/git/checkouts/maghemite-de41bdd6c14939ab/056283e/Cargo.toml
loading metadata for workspace crucible from /home/dap/.cargo/git/checkouts/crucible-f3b5bdecdc6486d6/b7b9d56/Cargo.toml
loading metadata for workspace dendrite from /home/dap/.cargo/git/checkouts/dendrite-627b239a911c6241/4cdc7d7/Cargo.toml
note: ignoring Cargo dependency from crucible-pantry -> ... -> crucible-control-client
note: ignoring Cargo dependency from omicron-sled-agent -> dns-server
Error: error: found no producer for API with client package name "clickhouse-admin-keeper-client" in any deployment unit (should have been one that contains server package "clickhouse-admin-api")

[sunshowers]

Looks good!

[sunshowers]

What does None mean?

[davepacheco]

Updated the comment to reflect that None means an API is deployed on real systems.

[sunshowers]

Ah thanks. Worth using serde's default for this?

[davepacheco]

We definitely could. I don't have a strong feeling either way.