Add an option `BrokerdPosition.bs_mktid` field

Such that backends can deliver their own internal unique
`MktPair.bs_mktid` when they can't seem to get it right via the
`.fqme: str` export.. (COUGH ib, you piece of sh#$).

Also add todo for possibly replacing the msg with a `Position.summary()`
"snapshot" as a better and more rigorously generated wire-ready msg.

piker/clearing/_messages.py

@@ -301,6 +301,9 @@ class BrokerdError(Struct):
 
 # TODO: yeah, so we REALLY need to completely deprecate
 # this and use the `.accounting.Position` msg-type instead..
+# -[ ] an alternative might be to add a `Position.summary() ->
+#   `PositionSummary`-msg that we generate since `Position` has a lot
+#   of fields by default we likely don't want to send over the wire?
 class BrokerdPosition(Struct):
     '''
     Position update event from brokerd.
@@ -313,3 +316,4 @@ class BrokerdPosition(Struct):
     avg_price: float
     currency: str = ''
     name: str = 'position'
+    bs_mktid: str|int|None = None