http4s: add ember constructor

[Fristi]

Ember uses keyed-pool which in turn will support otel4s which allows you track close connection pool metrics. As well Ember will provide a scala native version for which support can be added later.

I've removed the constructors with ExecutionContext as the BlazeClientBuilder says that this constructor is deprecated

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]

Thanks! MiMA only applies to core, as any libraries or integrations should not depend on concrete backend implementations.

Maybe we should include an optional dependency on ember, then, providing similar convenience methods to create an ember-backed-backend? Or at least providing a copy-pastable example of what to do to create a backend.

[Fristi]

Updated

[adamw]

Thanks! I've done some adjustments in bc50f9b