Create subscription version

Edit a Subscription by committing a new immutable Version that becomes current. It pins the current Version of the Subscription's own Saved Query (422 unknown_saved_query otherwise), an evaluator and a destination. It takes effect from its commit, recorded as its activation position with subscription.updated in every Corpus of the previous and new scope. Each later change is judged by the new Version, each earlier one by the Version effective before it. Nothing is backfilled and Matches keep the Version that produced them. Enabled state is unchanged. The evaluator, expression and configuration are checked as on creation (422 unsupported_evaluator, invalid_expression or invalid_subscription_configuration). Replay of the same key and request returns the same Version; a changed request is 409 idempotency_conflict. A deleted Subscription is 409 subscription_deleted.