sql: add stub exec.Factory for opt-driven distsql planning

This commit adds a stub implementation of `exec.Factory` interface that
will be creating DistSQL processor specs directly from `opt.Tree`,
sidestepping intermediate `planNode` phase.

It also introduces a new private cluster setting
"sql.defaults.experimental_distsql_planning" as well as a session
variable "experimental_distsql_planning" which determine whether the new
factory is used, set to `off` by default (other options are `on` and
`always` - the latter is only for the session variable).

`Off` planning mode means using the old code path, `on` means attempting
to use the new code path but falling back to the old one if we encounter
an error, and `always` means using only the new code path and do not
fallback in case of an error. Currently the fallback doesn't occur with
`always` only for SELECT statements (so that we could run other
statements types, like SET).

Release note: None

pkg/sql/distsql_running.go

@@ -1076,14 +1076,14 @@ func (dsp *DistSQLPlanner) PlanAndRunCascadesAndChecks(
 		}
 
 		evalCtx := evalCtxFactory()
-		execFactory := makeExecFactory(planner)
+		execFactory := newExecFactory(planner)
 		// The cascading query is allowed to autocommit only if it is the last
 		// cascade and there are no check queries to run.
 		if len(plan.checkPlans) > 0 || i < len(plan.cascades)-1 {
 			execFactory.disableAutoCommit()
 		}
 		cascadePlan, err := plan.cascades[i].PlanFn(
-			ctx, &planner.semaCtx, &evalCtx.EvalContext, &execFactory, buf, buf.bufferedRows.Len(),
+			ctx, &planner.semaCtx, &evalCtx.EvalContext, execFactory, buf, buf.bufferedRows.Len(),
 		)
 		if err != nil {
 			recv.SetError(err)