Add `__repr__` for connection pools

[rattrayalex]

• Initially raised as discussion Public API for accessing connection pool details httpx#2966 (reply in thread)

For example (proposed by @tomchristie):

>>> client = httpx.Client()
>>> client.transport
<httpx.HTTPTransport [Connections: 0 active, 0 keep-alive. Requests: 0 active, 0 queued.]>

Ideally, this will be followed by a public API to access this information programatically.

(Note that I am filing this issue as requested, but do not expect to use it myself, as I am interested primarily in the programmatic access).

cc @RobertCraigie