Investigate configuration option to enable read-through / no-op caches about aldica

[AFaust]

As part of #38 the case of the permissionEntitySharedCache came up. This cache uses PermissionEntity not just as the value but also as the value key. The class' equals/hashCode are only partially implemented - just to allow the class to be used as the value key. This now conflicts with how Ignite performs lookups for serialised keys. Since it cannot use the hashCode/equals implementation, it checks the serial state, and since full values are used as keys, cannot perform a lookup that only uses a partial entity, as this would not "equal" the actual key.
Luckily, the permissionEntitySharedCache is already pre-defined as a NullCache in Alfresco Spring XML. But this case has reinforced the need for an explictly configurable cache type as a means so that we (as the aldica module) can protect us against invalid / illegal use cases by Alfresco that we cannot otherwise fix / patch (without extreme effort).

As part of #38, I am effectively implementing variant 3 from the original list, without turning pre-existing NullCache instances into actual instantiation calls. Variant 1 I consider to be moot specifically for the warning note I originally added - we cannot simply disable all caches from our end, especially as we do not know what kind of caches might be added by 3rd party addons. Variant 2 has no place in our module and should be added as a cross-cutting feature in Alfresco core instead.
Turning pre-defined NullCache into actual instantiation calls would require that we also (dynamically?) check and potentially generate default configuration to ensure these caches are still instantiated as NullCache if no explicit config exists, or we would run risk of causing issues with caches added by 3rd party addons. For the sake of simplicity, this aspect of variant 3 has been dropped for the implementation in context of #38.

[andreaskring]

Thanks for the update 👍