Added implementation + tests for context aware via injection.

[niekraaijmakers]

Makes the following possible:

`@Model(adaptables = {SlingHttpServletRequest.class, Resource.class}, defaultInjectionStrategy = DefaultInjectionStrategy.OPTIONAL)
public class SampleModel {

@Via(type= ContextAwareConfigResource.class, value = "config")
@ValueMapValue
private String propertyTest;

public String getPropertyTest() {
    return propertyTest;
}

}`

Use via to point to context aware config resource, to inject valuemap values.
Tested it locally and works fine with latest versions.
What do you think?

[stefanseifert]

there is already an a JIRA ticket for this topic https://issues.apache.org/jira/browse/SLING-7256

in your implementation you introduced a thread local to cache things resolved by this injector. i think this is a bad idea in this context. if a cache is required because caconfig retrieval is slow, it should be added to caconfig in general and not only to the injector. please not that SlingAdaptable and sling models itself has also some caching capabilities, we should not add another here.

but before going into those details there are open general questions where this type of extension should be placed.