split VaultConnector interface into clients per module

The connector interface has grown quite big and does not even cover all
potential APIs. We now extract functionality into submodules and group
them to handle each area in separate interfaces. Provide fluent access,
strip prefixes from methods and preserve a 1:1 migration path.

Examples:

* connector.unseal() => connector.sys().unseal()
* connector.readSecretVersion() => connector.kv2().readVersion()
* connector.createToken() => connector.token().create()
* connector.lookupAppRole() => connector.appRole().lookup()
* connector.transitHash() => connector.transit().hash()

pom.xml

@@ -3,7 +3,7 @@
 
     <groupId>de.stklcode.jvault</groupId>
     <artifactId>jvault-connector</artifactId>
-    <version>1.5.3-SNAPSHOT</version>
+    <version>2.0.0-SNAPSHOT</version>
 
     <packaging>jar</packaging>