Add convenience .one() to do single search match

[piercefreeman]

Queries of primary keys are implemented as single select() statements in iceaxe, since this matches what we actually send to the query engine. It's often somewhat of a special case, however, because you usually want to do an implicit limit 1 and you want to typehint the returning object as only a single value.

This PR adds support for a .one() modifier to queries that will:

• Verify that you are querying for a whole serialized model
• Inject a limit 1
• Assert that one value is actually returned at runtime
• Typehint the output as a single object instead of a list, which is normally used by exec

This allows clients to replace:

results = await db_connection.exec(select(MyModel).where(MyModel.id == my_id).limit(1))
if not results:
  raise ValueError()
result = results[0]

with:

result = await db_connection.exec(select(MyModel).where(MyModel.id == my_id).one())