Provide a mechanism to load low-level backends

Some code is required so early in the bootstrap process that it cannot be included in a normal module. By the time we get to the point we can call hooks, the code, usually classes, has to be already active. In particular, this applies to:

• Database classes (DatabaseConnection and friends)
• Configuration storage classes (ConfigStorageInterface)
• Caching classes (BackdropCacheInterface)

We need a well designed and documented way to create backend driver modules.