http: client connection pool with circuit breaker

When using the host-level client-side API, I would find convenient to have a circuit-breakable version of a client connection pool.
It could be something like

```
def cachedHostConnectionPoolWithBreaker[T](breaker: CircuitBreaker)(...) = ???
```

Pool flows return `Try[HttpResponse]` already, so they can wrap the CB exceptions.

The CB could be wired in `clientFlow` helper method and used when the `mapAsyncUnordered` is invoked. I can take care of the PR in case this is deemed useful.

Start by reading the host-level client-side API and the clientFlow helper, then inspect where mapAsyncUnordered invokes the connection-pool flows. Clarify the proposed circuit-breaker API and how breaker exceptions should be represented in the existing Try[HttpResponse] results. Done means an agreed circuit-breakable connection-pool design with the relevant behavior specified.

Written by the indexing model from the issue text.