doc: add distinct for aggregate functions

Summarize the changes

• doc: add distinct for aggregate functions

Ensure that

• New code is covered by unit and integration tests.
• Related issues would be automatically closed with gitlab's closing pattern (Closes #1, #2).
• [] Public modules are documented (check the rendered version with cargo doc --open`).
• (if PEST grammar is changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Cherry-pick to: none / 24.2 / 24.3
• Update sbroad submodule in picodata/picodata.
• (if EBNF grammar is changed) create a follow-up issue in picodata/docs.

doc/sql/query.ebnf

@@ -42,7 +42,7 @@ expression  ::= (table '.')? column
                | 'TO_CHAR' '(' expression',' format ')'
                | 'TRIM' '(' ((('LEADING' | 'TRAILING' | 'BOTH')? expression) | ('LEADING' | 'TRAILING' | 'BOTH')) 'FROM' expression ')'
                | 'CASE' expression? ('WHEN' expression 'THEN' expression)+ ('ELSE' expression)? 'END'
-aggregate   ::= ('AVG' | 'COUNT' | 'MAX' | 'MIN' | 'SUM' | 'TOTAL') '(' expression ')'
+aggregate   ::= ('AVG' | 'COUNT' | 'MAX' | 'MIN' | 'SUM' | 'TOTAL') '(' ( 'DISTINCT'?  expression ) ')'
                | 'GROUP_CONCAT' '(' expression ','  "'" string "'" ')'
 cast        ::= 'CAST' '(' expression 'AS' type ')'
 values      ::= 'VALUES'