Asof join support

[ea-rus]

ASOF join example:

SELECT *
FROM table_1
ASOF JOIN table_2 ON (symbol)

Used in:

• snowflake
• duckdb
• questdb
• clickhouse
• and others

Required by https://linear.app/mindsdb/issue/BE-608/asof-join-support

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mindsdb_sql_parser
__about__.py	10	10	0%	1–10
__init__.py	116	22	81%	43, 47, 93, 110, 113, 134–153, 160–161
lexer.py	265	21	92%	346, 348, 350, 362, 364, 366, 372–390
logger.py	19	4	79%	14, 17, 23, 26
parser.py	1044	28	97%	120, 124, 263, 288, 385, 571, 588, 612–613, 812, 866, 943, 1034, 1087, 1097, 1135–1136, 1165, 1176, 1259, 1325, 1364, 1400, 1902, 1910, 1963–1966
utils.py	46	4	91%	73–79
mindsdb_sql_parser/ast
base.py	36	8	78%	13, 28, 31, 46, 49–52
create.py	76	12	84%	22–30, 91–96
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	46	1	98%	79
mindsdb_sql_parser/ast/select
case.py	38	2	95%	19, 22
constant.py	36	1	97%	23
data.py	11	4	64%	10–12, 15, 19
identifier.py	63	8	87%	42, 79–84, 91
native_query.py	13	1	92%	25
operation.py	139	4	97%	56, 65, 177, 201
parameter.py	11	1	91%	10
select.py	97	3	97%	148–153
star.py	12	2	83%	8–9
TOTAL	3178	148	95%

Tests	Skipped	Failures	Errors	Time
279	0 💤	0 ❌	0 🔥	15.445s ⏱️