Split XJC code into separate eppbindings subproject (#3244)

The generated XML / XJC code pretty much never changes but because
everything is lumped together in one compile task, we had to recompile
this every time anything else in core/ changes. It's technically
possible to use a separate source set for the XJC code while having it
remain in core/, but splitting it into a separate subproject gives
better separation and makes the caching / dependency graph clearer.

Pretty much all, except for the caching configuration, of the new
xjc/build.gradle is taken from the previous core/build.gradle.

docs/code-structure.md

@@ -42,6 +42,8 @@ Within the Nomulus repository there are a few notable subprojects:
     registrar console frontend
 *   `proxy` contains code for the EPP proxy, which relays port 700 requests to
     the core EPP services
+*   `eppbindings` contains code for EPP XML marshalling/unmarshalling and generation
+    of the corresponding Java classes
 *   `core` contains the bulk of the core Nomulus code, including request
     handling+serving, backend, actions, etc