feat: Update DMS provider to expect multiple sortable properties.

The `sortby` mechanism in the query method has been updated to expect sortable properties in a collection rather than as a single item, enabling multiple sortable properties. This has been done by adding a new `sortables` list member to the `MSCDMSCoreAPIProvider` class. This attribute is set to a `sortables` definition if it exists as a list in `msc-pygeoapi-config.yml`. Otherwise, it stores just the `time_field`.

msc_pygeoapi/provider/msc_dms.py

@@ -100,6 +100,9 @@ def __init__(self, provider_def):
         LOGGER.debug('Grabbing field information')
         self.fields = self.get_fields()
 
+        self.sortables = [sortable for sortable
+                          in provider_def.get('sortables', [self.time_field])]
+
     def get_fields(self):
         """
          Get provider field information (names, types)
@@ -243,8 +246,9 @@ def query(
             sort_by_values = []
             for sort in sortby:
                 # only allow sort on time_field
-                if sort['property'] != self.time_field:
-                    msg = f'Sorting only enabled for {self.time_field}'
+                if sort['property'] not in self.sortables:
+                    msg = f'Sorting only enabled for {self.sortables}'
+                    LOGGER.error(msg)
                     raise ProviderQueryError(msg)
                 LOGGER.debug(f'processing sort object: {sort}')
                 sort_property = f'{sort["order"]}properties.{sort["property"]}'