Help adding parser support for stream

[derekperkins]

Currently the only way to subscribe to messages via SQL is to run stream * from msg_tbl. It seems like it should be trivial to add support to the parser for column selection, stream col1, col2 from msg_tbl for someone with experience with yacc. Once there is parser support, I can make sure that it plumbs through messages properly. cc @systay
https://github.com/vitessio/vitess/blob/master/go/vt/sqlparser/sql.y#L413-L417

#5882

[nstandif]

select_expression was used instead of select_expression_list in both the AST and the yacc. You'd need to change both in order to plumb it out correctly.

https://github.com/vitessio/vitess/blob/master/go/vt/sqlparser/sql.y#L413-L417

stream_statement:
  STREAM comment_opt select_expression_list FROM table_name
  {
    $$ = &Stream{Comments: Comments($2), SelectExprs: $3, Table: $5}
  }

https://github.com/vitessio/vitess/blob/master/go/vt/sqlparser/ast.go#L83-L87

	Stream struct {
		Comments   Comments
		SelectExprs SelectExprs
		Table      TableName
	}