Extend Select Handler to support queries with UNION

There are number of execution pushdown handlers in MDB as of 10.5 however non of them supports UNION in the queries.
Select Handler(SH) should be extended if needed and used to pushdown execution to engines that are capable to process UNION.
SH now searches for a handler capable of executing the query during execute phase. in SELECT_LEX::find_select_handler(). This procedure could be applied to all SELECT_LEX_UNIT involved to search for an engine.