Backport CalCat CCV metadata API from EXtra

src/cal_tools/calcat_interface2.py

 import json
 import re
 from collections.abc import Mapping
-from dataclasses import dataclass, replace
+from dataclasses import dataclass, field, replace
 from datetime import date, datetime, time, timezone
 from functools import lru_cache
 from pathlib import Path
@@ -244,6 +244,8 @@ class SingleConstant:
     dataset: str
     ccv_id: Optional[int]
     pdu_name: Optional[str]
+    _metadata: dict = field(default_factory=dict)
+    _have_calcat_metadata: bool = False
 
     @classmethod
     def from_response(cls, ccv: dict) -> "SingleConstant":
@@ -252,6 +254,8 @@ class SingleConstant:
             dataset=ccv["data_set_name"],
             ccv_id=ccv["id"],
             pdu_name=ccv["physical_detector_unit"]["physical_name"],
+            _metadata=ccv,
+            _have_calcat_metadata=True,
         )
 
     def dataset_obj(self, caldb_root=None) -> h5py.Dataset:
@@ -266,6 +270,36 @@ class SingleConstant:
     def ndarray(self, caldb_root=None):
         return self.dataset_obj(caldb_root)[:]
 
+    def _load_calcat_metadata(self, client=None):
+        client = client or get_client()
+        calcat_meta = client.get(f"calibration_constant_versions/{self.ccv_id}")
+        # Any metadata we already have takes precedence over CalCat, so
+        # this can't change a value that was previously returned.
+        self._metadata = calcat_meta | self._metadata
+        self._have_calcat_metadata = True
+
+    def metadata(self, key, client=None):
+        """Get a specific metadata field, e.g. 'begin_validity_at'
+
+        This may make a request to CalCat if the value is not already known.
+        """
+        if key not in self._metadata and not self._have_calcat_metadata:
+            if self.ccv_id is None:
+                raise KeyError(f"{key!r} (no CCV ID to request data from CalCat")
+            self._load_calcat_metadata(client)
+
+        return self._metadata[key]
+
+    def metadata_dict(self, client=None):
+        """Get a dict of available metadata
+
+        If this constant didn't come from CalCat but we have a CalCat CCV ID,
+        this will fetch metadata from CalCat.
+        """
+        if (not self._have_calcat_metadata) and (self.ccv_id is not None):
+            self._load_calcat_metadata(client)
+        return self._metadata.copy()
+
 
 def prepare_selection(
     module_details, module_nums=None, aggregator_names=None, qm_names=None