Use pekko http connection pool

[ray-roestenburg-da]

We're currently using Http().outgoingConnectionUsingContext in HttpClient so that we can use a completionTimeout to implement a simple request-timeout.
Investigate alternatives to request-timeout, that still make retries work effectively when requests take too long.
It's important to note that long running requests are not advised to run on a pool: https://pekko.apache.org/docs/pekko-http/current/client-side/pool-overflow.html so we'll likely have to differentiate usage