UNION can be an expression

[ea-rus]

UNION can be an expression (the same way the SELECT does)
It allows to use it in IN conditions

Fixes https://linear.app/mindsdb/issue/FQE-1959/union-all-does-not-work-when-used-in-the-where-subquery

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mindsdb_sql_parser
__about__.py	10	10	0%	1–10
__init__.py	119	23	81%	44, 48, 53, 98, 115, 118, 139–158, 165–166
lexer.py	287	21	93%	378, 380, 382, 394, 396, 398, 404–422
logger.py	19	4	79%	14, 17, 23, 26
parser.py	1126	30	97%	129, 133, 305, 330, 436, 623, 640, 664–665, 892, 946, 1023, 1114, 1170, 1180, 1249, 1260, 1343, 1419, 1458, 1494, 1690–1691, 1870–1871, 2047, 2055, 2108–2111
utils.py	100	12	88%	73–79, 124, 140, 142, 144, 150–153, 161
mindsdb_sql_parser/ast
base.py	36	5	86%	13, 28, 31, 46, 51
create.py	82	9	89%	7–8, 23–31, 95
drop.py	52	2	96%	10, 13
insert.py	63	4	94%	39–41, 46
show.py	48	1	98%	18
update.py	53	5	91%	40–42, 75–76
mindsdb_sql_parser/ast/mindsdb
knowledge_base.py	127	3	98%	79, 118, 128
mindsdb_sql_parser/ast/select
case.py	37	1	97%	22
constant.py	36	1	97%	23
data.py	11	4	64%	10–12, 15, 19
identifier.py	83	11	87%	56, 104–112, 122
native_query.py	13	1	92%	25
operation.py	139	4	97%	57, 66, 178, 202
parameter.py	15	1	93%	20
select.py	108	3	97%	160–165
star.py	12	2	83%	8–9
TOTAL	3499	157	96%

Tests	Skipped	Failures	Errors	Time
323	0 💤	0 ❌	0 🔥	11.425s ⏱️