feat(hooks): add OnTxBegin and runInTx for resolvespec and restheadspec

Every transaction the handlers open now fires OnTxBegin first, with the
transaction in hookCtx.Tx, via common.RunRequestTx.

pkg/resolvespec/hooks.go

@@ -41,6 +41,12 @@ const (
 	// Unlike BeforeHandle, which fires once before operation dispatch, BeforeOp fires at each
 	// individual SQL-operation hook point, so it runs once per statement executed.
 	BeforeOp HookType = "before_op"
+
+	// OnTxBegin fires once, first, inside every transaction the handler opens
+	// (including the second short transaction for post-commit work). hookCtx.Tx
+	// is the transaction; use it to stamp transaction-local state such as RLS
+	// settings. An error or abort rolls the transaction back.
+	OnTxBegin HookType = common.TxHookName
 )
 
 // HookContext contains all the data available to a hook
@@ -76,6 +82,9 @@ type HookContext struct {
 	Tx common.Database
 }
 
+// SetTx points the context at the transaction in use (common.TxContext).
+func (c *HookContext) SetTx(tx common.Database) { c.Tx = tx }
+
 // HookFunc is the signature for hook functions
 // It receives a HookContext and can modify it or return an error
 // If an error is returned, the operation will be aborted