Add support for map proto fields for service registry

[bkrahmer]

The problem field looks like:
map<string, OtherMessageType> metadata = 10;

18:39:53.248 [Thread-287] INFO c.s.s.f.s.mockservice.MessageHandler - Starting message handler for com.sixt.service.zoning on port 42378
18:39:53.261 [pool-49-thread-1] ERROR c.s.s.f.r.consul.RegistrationManager - Error inspecting handlers
java.lang.NoSuchMethodException: com.google.protobuf.MapEntry.()
at java.lang.Class.getConstructor0(Class.java:3082)
at java.lang.Class.getDeclaredConstructor(Class.java:2178)
at com.sixt.service.framework.registry.consul.RegistrationManager.getProtobufClassFieldDescriptions(RegistrationManager.java:320)
at com.sixt.service.framework.registry.consul.RegistrationManager.getProtobufClassFieldDescriptions(RegistrationManager.java:341)
at com.sixt.service.framework.registry.consul.RegistrationManager.getProtobufClassFieldDescriptions(RegistrationManager.java:341)
at com.sixt.service.framework.registry.consul.RegistrationManager.addEndpointsTags(RegistrationManager.java:306)
at com.sixt.service.framework.registry.consul.RegistrationManager.getRegistrationTags(RegistrationManager.java:248)
at com.sixt.service.framework.registry.consul.RegistrationManager.buildJsonRequest(RegistrationManager.java:203)
at com.sixt.service.framework.registry.consul.RegistrationManager.attemptRegistration(RegistrationManager.java:133)
at com.sixt.service.framework.registry.consul.RegistrationManager.run(RegistrationManager.java:106)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)