client: Cache raw field information

The field JSON from the JIRA server contains important
information, such as schemas, which can be used by
applications building on this library. Since it's generally
preferred, whenever possible, to reduce unnecessary calls to
JIRA API endpoints, we can keep this information in the
client object.

jira/client.py

@@ -672,6 +672,14 @@ def __init__(
             JIRA.checked_version = True
 
         self._fields_cache_value: dict[str, str] = {}  # access via self._fields_cache
+        self._fields_cache_value_raw: list[dict[str, Any]] = []  # access via self._fields_cache_raw
+
+    @property
+    def _fields_cache_raw(self) -> list[dict[str, Any]]:
+        """Cached raw dictionary of of /field endpoint. Lazy loaded."""
+        if not self._fields_cache_value_raw:
+            self._fields_cache_value_raw = self.fields()
+        return self._fields_cache_value_raw
 
     @property
     def _fields_cache(self) -> dict[str, str]:
@@ -683,7 +691,7 @@ def _fields_cache(self) -> dict[str, str]:
     def _update_fields_cache(self):
         """Update the cache used for `self._fields_cache`."""
         self._fields_cache_value = {}
-        for f in self.fields():
+        for f in self._fields_cache_raw:
             if "clauseNames" in f:
                 for name in f["clauseNames"]:
                     self._fields_cache_value[name] = f["id"]