Run datastore tests against multiple schemas

We want to prove in tests that some Janus version can run safely on
multiple schema versions to make database schema migrations safer. In
this commit:

 - `aggregator_core::test_util::EphemeralDatastore` can now be
   constructed with a `max_schema_version` argument to control which
   migration scripts are applied during tests
 - `datastore::SUPPORTED_SCHEMA_VERSIONS` is now emitted by the
   `supported_schema_versions` macro, which also constructs a template
   for stamping out tests.
 - We adopt [`rstest`][1] to inject an `EphemeralDatastore` instance
   into tests in `aggregator_core::datastore::Datatore::tests`. Using
   [`rstest_reuse`][2], we can automatically stamp out versions of
   existing tests that run using multiple schema versions.

We only use this dependency injection technique in the `datastore`
module, because that should be the only module that's tightly coupled to
the database schema. We could run all tests that use a datastore this
way, at the cost of increasing overall test runtime.

[1]: https://docs.rs/rstest
[2]: https://docs.rs/rstest_reuse

aggregator_core/Cargo.toml

@@ -62,9 +62,12 @@ uuid = { version = "1.3.1", features = ["v4"] }
 
 [dev-dependencies]
 assert_matches = "1"
+async-std = { version = "1.12.0",  features = ["attributes"] }
 hyper = "0.14.26"
 janus_aggregator_core = { path = ".", features = ["test-util"] }
 janus_core = { workspace = true, features = ["test-util"] }
+rstest = "0.17.0"
+rstest_reuse = "0.5.0"
 serde_test = "1.0.160"
 tempfile = "3.5.0"
 tokio = { version = "1", features = ["test-util"] }  # ensure this remains compatible with the non-dev dependency