Add ActionSerializerInjector

Add ActionSerializerInjector

Add injector where all action serializers will be registered and
call it from general SerializerInjector.
Add AbstractActionSerializer and AbstractSetFieldActionSerializer to be able
to actually create action serializers.
Add unit tests for ActionSerializerInjector
Add ActionUtil#writeAction that tries to determine what type of
action we are serializing, and serializes it.

See also: bug 7138

Change-Id: I25549b453a0a36d38f9b42de7b8087a41d438f7b
Signed-off-by: Tomas Slusny <tomas.slusny@pantheon.tech>