Add event dispatcher to OC\DB\QueryBuilder::execute

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Is your feature request related to a problem? Please describe.
For advanced app integration, it would be nice to have an event dispatcher run before and after OC\DB\QueryBuilder::execute. While this may seem a little odd, there are certainly situations where this functionality would be useful:

• Security apps could check SQL queries and parameters for dangerous patterns and prevent them from executing.
• Some apps don't fire events themselves, but they do access the DB. A QueryBuilder event listener would allow developers to execute code when a query matching certain conditions is executed.
• Deep integration with the platform and other apps would be possible.
• In my particular case, I'm wanting to encrypt/decrypt certain fields used by other apps. I have written an app to transparently encrypt DAV data, but this required some rather kludge code to avoid editing core libraries. An event listener would have been a simpler solution.

Describe the solution you'd like
It would be great to have an event dispatcher added to OC\DB\QueryBuilder::execute both before and after $result = $this->queryBuilder->execute();. The dispatcher should be able to check if propagation has been halted, and if so, it will return the result provided by the listener.

Describe alternatives you've considered

• I have tried to use the existing middleware and events to add certain functionalities, but these were neither efficient, reliable, nor ideal.
• In order to modify DB calls made by existing apps, I have "overloaded" their classes by unregistering their existing classmap, autoloading a modified classmap specifying my modified classes, and executed queries this way. While this works and doesn't modify core code . . . it is gross and not recommended.
• I have tried editing core libraries, but this is obviously not recommended.

Start at OC\DB\QueryBuilder::execute and inspect how its underlying queryBuilder executes SQL. Review the existing middleware and events mentioned in the issue before deciding how pre- and post-execution dispatching and halted propagation should work. Done means listeners can inspect or stop a query before execution and receive or provide the result afterward.