How to build AnnotationOverrides from ObjectMapper

Hi Tatu,

In the context of my JSON Schema Generator library, I've been asked to support the mix-ins registered on a given Jackson `ObjectMapper`, when resolving members via classmate.
I saw the usage of the `AnnotationOverrides` type in classmate expecting this method to be implemented:
```java
List> mixInsFor(ClassKey);
```
Unfortunately, the `ClassKey` is a utility type, which doesn't offer a getter for the wrapped `Class`, which complicates things.

Likewise on the Jackson `ObjectMapper`, there is no way to retrieve a full set of registered mix-ins as a simple `Map`. Just this method:
```java
Class findMixInClassFor(Class)
```

Any suggestions on how to implement the `AnnotationOverrides` based on a fully configured `ObjectMapper` without requiring the same mix-ins to be provided all over again?