parse timestamps as ZonedDateTime internally

Timestamps have been stored with their String representation from the
API with convenience methods to convert them into ZonedDateTime.
We now use the Jackson JavaTimeModule to parse them directly and swap
the real and convenience getters.

src/main/java/module-info.java

@@ -33,4 +33,5 @@ module de.stklcode.jvault.connector {
     requires java.base;
     requires java.net.http;
     requires com.fasterxml.jackson.databind;
+    requires com.fasterxml.jackson.datatype.jsr310;
 }