parse_functions: find functions inside FROM-clause subqueries, joins and table-function arguments

[asubbarao]

parse_function_names and parse_functions skip everything in the FROM clause: subqueries, both sides of a join, and table-function arguments, as well as scalar subqueries. The community build (parser_tools pinned at 6a94f2b) returns [] for the first query below.

SELECT parse_function_names('SELECT * FROM (SELECT avg(n) AS a FROM range(5) t(n)) s');
-- before: []    after: [avg]

The traversal now descends into from_table (join sides and conditions, subquery references, table-function arguments and subqueries) and into SubqueryExpression, following the same table-reference pattern parse_tables uses.

query	before	after
FROM (SELECT avg(n) …) s	[]	[avg]
FROM (SELECT upper(x) …) a JOIN (SELECT lower(y) …) b	[]	[upper, lower]
FROM range(abs(-5))	[]	[abs]
SELECT (SELECT sum(1)) WHERE x IN (SELECT length(z) …)	[]	[sum, length]

Verified against DuckDB v1.4.4 (this repo's pin): the four cases are added to both parse_function_names.test and parse_functions.test. On the unpatched source both files fail ([] <> [avg], 2 of 11 test cases); with the patch all 11 test cases pass.

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