Implement client connection pooling

[jdegoes]

Is your feature request related to a problem? Please describe.

The performance of the Http client can be improved through connection pooling.

Describe the solution you'd like

Connection pooling is easier to implement with a keyed pool, which ZIO 2 does not yet support.

We can build a ZKeyedPool here and upstream to ZIO 2 later:

trait ZKeyedPool[+Error, -Key, Item] {
  def get(key: Key)(implicit trace: Trace): ZIO[Scope, Error, Item]

  def invalidate(item: Item)(implicit trace: Trace): UIO[Unit]
}
object ZKeyedPool {
  def makeWith[K, R, E, A](get: K => ZIO[R, E, A], total: => Range, maxPerKey: => Int)(implicit trace: Trace): ZKeyedPool[E, K, A] = ???
}

Then we can utilize this connection pool in the implementation of Client to ensure that we have ready-to-use connections to common targets.

Describe alternatives you've considered

None.