Boost: Conflict detection for the same features in other plugins

We're getting a lot of requests for conflicts between Boost and Litespeed or WP Rocket or similar optimization plugins.

For example, Litespeed has Page Optimization features that overlap Boost's. Primarily Defer JS, Critical CSS, JS/CSS concatenation.

We should show notices when a conflict occurs. Either when a user turns on a feature and Boost and a similar feature is enabled in another plugin, or when the feature is enabled on Litespeed's end (I think we can detect this).

We should do this for more popular alternatives to Boost.

The deliverable would be similar to what Boost Cache does when it detects conflicts on attempted activation.

Start by reviewing the existing Boost Cache conflict-detection behavior and the overlapping features named here: Defer JS, Critical CSS, and JS/CSS concatenation. Done means Boost shows notices when these features conflict with Litespeed, WP Rocket, or other popular optimization plugins, including conflicts detected after activation.

Written by the indexing model from the issue text.