Load engines properly in both Zeitwerk and Classic modes

[paracycle]

Motivation

Until now, our engine loading in Classic mode has been super sketchy, and even our engine loading in Zeitwerk mode was missing some functionality.

This PR improves engine loading so that it:

1. works both in Classic and Zeitwerk modes,
2. properly runs engine initializers,
3. is best effort for a given engine,
4. isolates engines from failures in loading of other engines.

By running engine initializers, we are able to finally fix #557. However, the fact that we run initializers means that some engines could raise exceptions during load time depending on the existence of certain conditions. For example, Active Storage raises an exception when ActiveStorage::Blob is loaded if it is not configured. This means that applications that have not configured AS will only get abridged RBI file, since we won't be able to load anything past ActiveStorage::Blob.

This is not a problem, though, since if the application has not configured Active Storage, it means they are not really using it anyway, so it should not matter. For an application that has a proper configuration of AS, they would get the full RBI file generated.

Implementation

Mimic the behaviour of running initializers and eager loading of engines in classic mode and in zeitwerk mode

Tests

Made the Zeitwerk mode test explicit and added a Classic mode test.

[egiurleo]

This is so exciting! I mostly have questions and one small comment fix 🕺🏻

[egiurleo]

🎉