Move Nexus notification to standalone task (#1584)

Previously, we would spawn a new `tokio::task` in the `Downstairs` to do
an async notification to Nexus, iff the `notify-nexus` feature was
enabled. This is awkward for a few reasons:

- Spawning tasks in an otherwise-synchronous function can be surprising
(since it implicitly requires a tokio runtime)
- It requires importing a bunch of Nexus types into the `mod downstairs`
- This code was only compiled if `notify-nexus` is enabled, which means
that it's easy to have something that builds locally but fails in CI
- There's a bunch of duplicate logging, because each function notifies
Nexus separately

This PR moves Nexus notifications to a separate `notify` module, which
runs a single task to do these notifications in one place. It
communicates to the rest of Crucible through a queue; if the queue fills
up, we log and discard the notification request (since this is
best-effort).

All of this code is _always_ compiled; we check the feature (using
`cfg!(feature = "notify-nexus")` to determine whether it's actually
enabled. This module is responsible for translating into Nexus types, so
they don't leak into the rest of the codebase.

upstairs/Cargo.toml

@@ -12,7 +12,7 @@ path = "src/lib.rs"
 
 [features]
 asm = ["usdt/asm"]
-notify-nexus = ["nexus-client", "internal-dns", "progenitor-client", "http", "omicron-uuid-kinds"]
+notify-nexus = []
 integration-tests = []
 
 [dependencies]
@@ -56,11 +56,9 @@ aes-gcm-siv.workspace = true
 rand_chacha.workspace = true
 reqwest.workspace = true
 crucible-workspace-hack.workspace = true
-nexus-client = { workspace = true, optional = true }
-internal-dns = { workspace = true, optional = true }
-omicron-uuid-kinds = { workspace = true, optional = true }
-progenitor-client = { workspace = true, optional = true }
-http = { workspace = true, optional = true }
+nexus-client.workspace = true
+omicron-uuid-kinds.workspace = true
+internal-dns.workspace = true
 
 [dev-dependencies]
 expectorate.workspace = true

upstairs/src/client.rs

@@ -1919,7 +1919,7 @@ impl DownstairsClient {
         self.client_delay_us.load(Ordering::Relaxed)
     }
 
-    #[cfg(feature = "notify-nexus")]
+    /// Looks up the region UUID
     pub(crate) fn id(&self) -> Option<Uuid> {
         self.region_uuid
     }