SSDM-13251: Implemented login, isSessionActive, logout methods for client. Added tests

api-data-store-server-java/build.gradle

...src/main/java/ch/ethz/sis/afsclient/client/AfsClient.java

...test/java/ch/ethz/sis/afsclient/client/AfsClientTest.java

...st/java/ch/ethz/sis/afsclient/client/DummyHttpServer.java

server-data-store/build.gradle

...ava/ch/ethz/sis/afsserver/http/impl/NettyHttpHandler.java

...java/ch/ethz/sis/afsserver/http/impl/NettyHttpServer.java

...a/ch/ethz/sis/afsserver/server/impl/ApiServerAdapter.java

...re/src/test/java/ch/ethz/sis/afsserver/ApiClientTest.java