added hook to allow handling of custom metadata and insertion of custom ...

...InjectionPoints

This would allow framework developers to leverage the Injector class's reflection routines, and avoid repeating them if they wish to add custom metadata.

With the hook, framework developers can then process any custom metadata and create and insert InjectionPoints before the PostConstructInjectionPoints are added to the InjecteeDescription.

Without the hook there is a considerable amount of duplicated effort required to achieve this.

The extra method call does add a small overhead, so perhaps some thought could be put into that before accepting it into master branch.

I have an example of usage if anyone would like to see what kind of possibilities it opens up.