Sql model outputs query needs better way to define version

[kartik4949]

current way of model outputs retrieval in sql is

table.select(...).outputs(x='model_linear_a/0')

basically
key = '{model_name}/{version}'

we should define it better and document

[jieguangzhou]

Maybe we can unify mongodb and ibis as

query: select.outputs(field=“x”, model=“model_linear_a”, version=0 / None)
outputs: document.outputs(field=“x”, model=“model_linear_a”, version=0 / None)

WDYT? @kartik4949