Implement PDOEngine::initialize

Summary:
If a project needs Keruald Database with PDO support, it would benefit
from calling PDOEngine::initialize to get an instance of PDOEngine
instead of DatabaseEngine.

It also throws EngineSetupException if a non-PDO engine is configured.

Ref T2169

Test Plan: Tested with D3835 on Obsidian codebase

Maniphest Tasks: T2169

Differential Revision: https://devcentral.nasqueron.org/D3837