Add features to monolithic_integration_test

[divergentdave]

This moves most of monolithic_integration_test and its novel dependencies behind a new "test-util" feature flag, similar to the other
crates. With this, our release builds will no longer have the feature flags janus_core/test-util, janus_server/test-util, nor reqwest/default-tls set, due to crate feature flags being unified across the workspace. As a result, we can once again eliminate OpenSSL as a build-time dependency.

I also fixed a tangentially related inaccuracy in some comments that Tim pointed out.

[branlwyd]

While I don't mind doing this as a workaround, we really shouldn't have to introduce a test-util feature on a crate that is already test-only just to keep Cargo from bleeding its dependencies into other, unrelated crates. Every user of monolithic_integration_test is going to want to specify the test-util feature!

Could we instead factor this as janus & daphne features? (The former would enable use of the janus module, the latter the daphne module.) This would make usage of a feature more reasonable. And I suppose I exposed this functionality in the first place with the thought that others may want to build integration tests of their own against Janus; such a user would want to enable the janus feature but not the daphne feature, I suppose.

[branlwyd]

I think the specific condition is if the cluser is identified by IP, rather than DNS name -- IIUC rustls does not yet support IP-address-based certs.

Maybe "Kubernetes clusters identified by IP" or "local Kubernetes clusters" instead of "most Kubernetes clusters"? Both are more specific to the conditions that cause breakage.