Propagate cancellation for http4s backend

[cwe-dixa]

Previously, cancelling a request using the http4s backend did not attempt to cancel the underlying request but instead returned instantly.

This PR changes the behavior to cancel the underlying request fiber if the "outer" request is cancelled.

Note that this means that where cancellation was essentially instant before, it now depends on however long it takes to cancel the request fiber. I would argue this fits better with cats-effect (cooperative cancellation and all), but it is a behavior change nonetheless.

Before submitting pull request:

• Check if the project compiles by running sbt compile
• Verify docs compilation by running sbt compileDocs
• Check if tests pass by running sbt test
• Format code by running sbt scalafmt

[adamw]

Yes, definitely that's the "cats-effect" way of doing things. Thanks!