add pre exec hook

[ccyphers]

I see there's a postProcessResponse hook but no capabilities for pre SQL execution actions. This PR allows the user to define preExecHook in the config where the value of preExecHook is a function that will be executed before the SQL is executed. The callback function executed has an option, continue that controls whether or not to continue the flow and execute the final SQL. There are cases where the callback function would want to set continue to false as it might be short circuiting in use cases such as cache HITS from other sources.