extends audit service to support adapters in EL expressions

*issue : with current audit service can NOT log values resulted from customized method calls

Audit Service use EL expression to add extra information to be logged.
For now there are 3 objects available in the context to be used in EL
expressions:

message - Document message describing the event
source - Document from which the event is from
principal - Identity of the event owner

<extension point="extendedInfo" target="org.nuxeo.ecm.platform.audit.service.NXAuditEventsService">
<extendedInfo expression="${source.myschema.myproperty}" key="extraInfo" />
</extension>

But sometimes we need to implement some logic to produce the value that we want to record
in our case
[...]
if (doc.isProxy()) {
CoreSession session = CoreInstance.getInstance().getSession(sessionId);
DocumentModel tmp = session.getSourceDocument(doc.getRef());
originalDocUUID = session.getSourceDocument(tmp.getRef()).getId();
}
else {
originalDocUUID = doc.getId();
}
[...]

• posbile solution
  added a new extension point to org.nuxeo.ecm.platform.audit.service.NXAuditEventsService called "docAdapter"

using this point will register an adapter for DocuementModel
<extension point="doc" target="org.nuxeo.ecm.platform.audit.service.NXAuditEventsService">
<adapter class="org.nuxeo.myadapter" name="myadapter" />
</extension>

class - is the qualified name of the class that will wrap the document model specified by "source"
name - is the name that will be used in EL

then win "extendedInfo" will add
<extension point="extendedInfo" target="org.nuxeo.ecm.platform.audit.service.NXAuditEventsService">
<extendedInfo expression="${myadapter.originalDocUUID}" key="original_doc_uuid" />
</extension>