agent: touch expanded specs for user-initiated publications

[psFried]

For user-initiated publications, we expand the draft in order to re-validated connected specs along with those that are being modified. With the introduction of touch publications, we now have the option to only touch expanded specs instead of incrementing their last_pub_id. This would help avoid unnecessary entries in publication_specs, and also prevents unnecessary ExpectPubIdNotMatched errors in scenarios where multiple publications concurrently affect connected specs (e.g. publication A of a collection and publication B of a connected materialization).