Relax the concept of extension point

[decebals]

These changes will solve some of the issues discussed in #358.
It contains:

• relax the type of Extension#points field (see Support any interface as an ExtensionPoint #289 (comment))
• ExtensionAnnotationProcessor checks that an extension implements the ExtensionPoint marker interface only if the pf4j.checkExtensionPoint annotation option is present. By default the check is disabled

[dmitry-timofeev]

LGTM, however, I am not an expert in this code 👍

[decebals]

Is there any interest in this functionality?

[dmitry-timofeev]

Is there any interest in this functionality?

Unfortunately, I won't be able to add much beyond discussion in #289, as I no longer work on a project that uses PF4J and not aware of any recent changes to PF4J, and how they might interact with this feature. The considerations in #289 seem valid to me.

[decebals]

@dmitry-timofeev Thanks! I want to see if other users see valuable this PR.