Add `custom_granularity_columns` to `TimeSpineSource` #3754

Job	Run time
MetricFlow Tests (PostgreSQL)	5m 10s
MetricFlow Packages Tests (3.8)	3m 41s
MetricFlow Tests (DuckDB) (3.8)	3m 7s
MetricFlow Packages Tests (3.12)	5m 3s
MetricFlow Tests (DuckDB) (3.12)	3m 6s
MetricFlow Unit Tests	1s
	20m 8s

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