Make requirements read-only for plugin developers

[sebinside]

Description

Because of the current API design, it possible to the plugin dev to edit or unset a retrieved requirement. This is not intended. The addRequirement should be reworked to only return a container with the requirement reference and possible getters.

References

https://github.com/codeoverflow-org/chatoverflow-api/blob/702cf575556b162875b808b5d55d166bb2250752/src/main/java/org/codeoverflow/chatoverflow/api/plugin/configuration/Requirements.java#L57

[sebinside]

Closed with a25d1c2 and codeoverflow-org/chatoverflow-api@3133d80.