Allow ledger passes to ignore (symcache) unknown fqmes

For example in the paper-eng, if you have a backend that doesn't fully
support a symcache (yet) it's handy to be able to ignore processing
other paper-eng txns when all you care about at the moment is the
simulated symbol.

NOTE, that currently this will still result in a key-error when you load
more then one mkt with the paper engine (for which the backend does not
have the symcache implemented) since no fqme ad-hoc query was made for
the 2nd symbol (and i'm not sure we should support that kinda hackery
over just encouraging the sym-cache being added?). Def needs a little
more thought depending on how many backends are never going to be able
to (easily) support caching..

piker/accounting/_pos.py

@@ -496,7 +496,7 @@ class Account(Struct):
 
         _mktmap_table: dict[str, MktPair] | None = None,
 
-        only_require: list[str]|None = None,
+        only_require: list[str]|True = True,
         # ^list of fqmes that are "required" to be processed from
         # this ledger pass; we often don't care about others and
         # definitely shouldn't always error in such cases.
@@ -549,7 +549,14 @@ class Account(Struct):
                 if _mktmap_table is None:
                     raise
 
-                required: bool = fqme in only_require
+                required: bool = (
+                    only_require is True
+                    or (
+                        only_require is not True
+                        and
+                        fqme in only_require
+                    )
+                )
                 # XXX: caller is allowed to provide a fallback
                 # mktmap table for the case where a new position is
                 # being added and the preloaded symcache didn't