[mssql] Parse CROSS/OUTER APPLY

T-SQL (and Oracle) support non-standard syntax, which is similar in
functionality to LATERAL joins in ANSI and PostgreSQL
<https://blog.jooq.org/tag/lateral-derived-table/>: it allows to use
the columns from the tables defined to the left of `APPLY` in the
"derived tables" (subqueries) to the right of `APPLY`. Unlike ANSI
LATERAL (but like Postgres' implementation), APPLY is also used with
table-valued function calls.

Despite them being similar, we represent "APPLY" joins with
`JoinOperator`s of its own (`CrossApply` and `OuterApply`). Doing
otherwise seemed like it would cause unnecessary confusion, as those
interested in dialect-specific parsing would probably not expect APPLY
being parsed as LATERAL, and those wanting to forbid non-standard SQL
would not be helped by this either.

This also renames existing JoinOperator::Cross -> CrossJoin to avoid
confusion with CrossApply.

tests/sqlparser_mssql.rs

@@ -52,6 +52,22 @@ fn parse_mssql_delimited_identifiers() {
     );
 }
 
+#[test]
+fn parse_mssql_apply_join() {
+    let _ = ms_and_generic().verified_only_select(
+        "SELECT * FROM sys.dm_exec_query_stats AS deqs \
+         CROSS APPLY sys.dm_exec_query_plan(deqs.plan_handle)",
+    );
+    let _ = ms_and_generic().verified_only_select(
+        "SELECT * FROM sys.dm_exec_query_stats AS deqs \
+         OUTER APPLY sys.dm_exec_query_plan(deqs.plan_handle)",
+    );
+    let _ = ms_and_generic().verified_only_select(
+        "SELECT * FROM foo \
+         OUTER APPLY (SELECT foo.x + 1) AS bar",
+    );
+}
+
 fn ms() -> TestedDialects {
     TestedDialects {
         dialects: vec![Box::new(MsSqlDialect {})],