WL#4627: Prepared Statements: add C API call mysql_stmt_execute_immediate()

Add a way to execute a statement without preparing it, and get results in binary
format.
For that, extend the client/server protocol with COM_STMT_EXECUTE_IMMEDIATE
command, and add a C API call mysql_stmt_execute_immediate().
The new protocol command is similar to COM_QUERY, except the results are sent
back in the binary format.
The results can be bound to client buffers and retrieved in the same way as
results of mysql_stmt_execute(). 
COM_STMT_EXECUTE_IMMEDIATE does not support multi-statements.