Add the ability to implement a specific logic when a capacity is enabled

[cedric-anne]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.

Description

It is based on #18306 and replaces it. It fixes #18304.

The purpose of this PR is to add a CapacityInterface::onCapacityEnabled() method that can be used to define a logic that has to be call only at the moment a capacity is enabled. For instance, the loading of the default import rules has to be done only once.

I had to review a bit the custom assets/dropdowns definition handling to be able to reload a definition at runtime. This was already done in the tests with some kind of hacky code, but now it is done in a more proper way, and this is no longer necessary to use the reflection API to do this.

I also added a call to CapacityInterface::onCapacityDisabled() in the AssetDefinition::cleanDbOnPurge(). It is probably unecessary for the most common cases, but it permits to be sure that the cleaning logic implemented by each capacity is executed when a definition is removed.

[trasher]

Tests are failing :/

[cedric-anne]

Tests are failing :/

It seems that registered autoloaders are not available on the test execution context. I had to register them on each test setup, see my last commit.