tp: allow pipelines as subqueries

[LalitMaganti]

Pipelines can only be whole statements or the body of a CREATE PERFETTO TABLE, so nothing written in SQL can use one: not a join, a CTE, a view or a function. That is what stops users like wattson moving off the interval intersection macros.

A pipeline in parentheses now reads like any other subquery, in a FROM clause or as a CTE:

SELECT t.id, p.total
FROM tree t
JOIN (FROM tree |> TREE ACCUMULATE UP SUM(self) AS total) p USING (id)

How:

• Once a statement is parsed, each such pipeline is compiled and replaced with SQL running its plan, recorded as a rewrite where it was written, like a macro call. So SQL and pipelines nest either way round, pipelines work in macros and macros in pipelines, and views, functions and triggers need nothing new.

• The SQL a pipeline reads is evaluated there too, so it can read a function's arguments.

• A pipeline's rewrite stands in for the macro calls written inside it, and errors trace back through macro calls.

• SQL reading a pipeline of the same statement is described by the plan that pipeline compiled to.

• A pipeline read by a pipeline is compiled into the same plan, in a scope of its own, so it can also be an interval intersection's operand.

• A pipeline can read the CTEs of the statement it is written in (via tp: let semantic analysis see CTEs #7643).

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[LalitMaganti]

Shelved for now: pipelines inside other SQL (subqueries, CTEs, joins) need proper predicate pushdown and cost estimation first. Pipelines reading SQL and CTEs as input continue in #7641 and #7643. Branch kept for when we pick this up again.