Port binance to `httpx`

Like other backends use the `AsyncClient` for all venue specific
client-sessions but change to allocating them inside `get_client()`
using an `AsyncExitStack` and inserting directly in the
`Client.venue_sesh: dict` table during init.

Supporting impl tweaks:
- remove most of the API client session building logic and instead make
  `Client.__init__()` take in a `venue_sessions: dict` (set it to
  `.venue_sesh`) and `conf: dict`, instead opting to do the http client
  configuration inside `get_client()` since all that code only needs to
  be run once.
 |_load config inside `get_client()` once.
 |_move session token creation into a new util func `init_api_keys()` and
  also call it from `get_client()` factory; toss in an ex. toml section
  config to the doc string.
- define `_venue_urls: dict[str, str]` (content taken from old static
  `.venue_sesh` dict) at module level and feed them as `base_url: str`
  inputs to the client create loop.
- adjust all call sigs in httpx-sesh-using methods, namely just
  `._api()`.
- do a `.exch_info()` call in `get_client()` to cache the symbology
  set.

Unrelated changes for various other outstanding buggers:
- to get futures feeds correctly loading when selected
  from search (like 'XMRUSDT.USDTM.PERP'), expect a `MktPair` input to
  `Client.bars()` such that the exact venue-key can be looked up (via
  a new `.pair2venuekey()` meth) and then passed to `._api()`.
- adjust `.broker.open_trade_dialog()` to failover to paper engine when
  there's no `api_key` key set for the `subconf` venue-key.

piker/brokers/ib/api.py

@@ -1250,6 +1250,12 @@ async def load_aio_clients(
 
         for i in range(connect_retries):
             try:
+                log.info(
+                    'Trying `ib_async` connect\n'
+                    f'{host}: {port}\n'
+                    f'clientId: {client_id}\n'
+                    f'timeout: {connect_timeout}\n'
+                )
                 await ib.connectAsync(
                     host,
                     port,
@@ -1367,7 +1373,9 @@ async def load_clients_for_trio(
     a ``tractor.to_asyncio.open_channel_from()``.
 
     '''
-    async with load_aio_clients() as accts2clients:
+    async with load_aio_clients(
+        disconnect_on_exit=False,
+    ) as accts2clients:
 
         to_trio.send_nowait(accts2clients)