feat: add metricflow_time_spine.yml (#52)

* feat: add metricflow_time_spine model configuration to fix warning Time spines without YAML configuration are in the process of
  deprecation. Please add YAML configuration for your 'metricflow_time_spine'
  model.

* fix: correct indentation in metricflow_time_spine YAML configuration

Author: IL-Jerry

models/04_metric/metricflow_time_spine.yml

@@ -0,0 +1,7 @@
+models:
+  - name: metricflow_time_spine
+    time_spine:
+      standard_granularity_column: date_day
+    columns:
+      - name: date_day
+        granularity: day