enforce use of builder to create a new HTTPVaultConnector (#54)

Remove constructors of HTTPVaultConnector and make the builder
constructor package-private to enforce use of .builder()....build()

For convenience we add direct builder constructors with a full URI
argument to allow a one-line initialization if necessary.

src/main/java/de/stklcode/jvault/connector/internal/RequestHelper.java

@@ -57,7 +57,7 @@ public final class RequestHelper implements Serializable {
                          final Integer timeout,
                          final String tlsVersion,
                          final X509Certificate trustedCaCert) {
-        this.baseURL = baseURL;
+        this.baseURL = baseURL + (baseURL.endsWith("/") ? "" : "/");
         this.retries = retries;
         this.timeout = timeout;
         this.tlsVersion = tlsVersion;