[MySQL, Oracle] Parse optimizer hints

[xitep]

this should resolve #1995

[iffyio]

@xitep wondering would it not make more sense to support features like these via the new parse_sql_with_comments API?

[xitep]

surely it would be one way. (the only premise for that would to be have the next non-comment token after the corresponding keywords covered with a span, like suggested in #2099.) however, when processing such statements, these hints are something you generally don't want to leave out, even when parsing without comments. hence this proposal to embed it directly into the AST.