Add filter to deactivate the plugin from the code

[rodrigo-arias]

This PR adds a new filter (disabled by default) to deactivate the plugin from the code.

Useful when Autoptimize is a dependency of another plugin installed in both standard and headless installations and you want to disable it only in the latter.

Originally discussed in https://wordpress.org/support/topic/filter-to-deactivate-the-plugin/

[rodrigo-arias]

@futtta thanks for merging this. Do you have an ETA for when you will release the next version?