Implement Helper async aggregation.

Implementation strategy: two new report aggregation states,
HelperInitProcessing & HelperContinueProcessing, are introduced. These
states represent a report aggregation which has received an
initialization/continuation message from the Leader, but has not yet
processed that message (and, in the case of continuation, the existing
state) into the next HelperInit/HelperContinue state.

The aggregation initialization/continuation endpoints now always
construct these new Processing states as part of initial request
handling. In the synchronous aggregation case, the Processing states are
processed into HelperInit/HelperContinue states before being written
back to durable storage. In the asynchronous aggregation case, the
Processing states are written back to storage immediately, and are later
processed by the aggregation job driver into the next
HelperInit/HelperContinue state.

An upshot of this is that a Helper deployment performing asynchronous
aggregation MUST now run an aggregation job driver.

The implementation is complete as far as the protocol is concerned.
However, there is no logic yet to implement setting the Retry-After
header, which is spec-optional but likely important to a production
deployment. To facilitate testing, a new aggregation job driver
parameter is introduced which controls how quickly the Leader should
poll for outstanding asynchronous aggregaiton jobs if the Retry-After
header is not set.

aggregator/Cargo.toml

@@ -29,14 +29,13 @@ test-util = [
     "janus_aggregator_core/test-util",
     "janus_core/test-util",
     "janus_messages/test-util",
-    "dep:assert_matches",
     "dep:testcontainers",
     "dep:trillium-testing",
 ]
 
 [dependencies]
 anyhow.workspace = true
-assert_matches = { workspace = true, optional = true }
+assert_matches = { workspace = true }
 async-trait = { workspace = true }
 aws-lc-rs = { workspace = true }
 backoff = { workspace = true, features = ["tokio"] }
@@ -70,6 +69,7 @@ postgres-protocol = { workspace = true }
 postgres-types = { workspace = true, features = ["derive", "array-impls"] }
 prio.workspace = true
 prometheus = { workspace = true, optional = true }
+querystring = { workspace = true }
 rand = { workspace = true, features = ["min_const_gen"] }
 rayon.workspace = true
 regex = { workspace = true }