[Aggregates] [Windowing] COVAR_POP uses COVAR_SAMP implementation

[ohaibbq]

Currently these functions return the same value. The population value needs to be updated to actually calculate population covariance.

		{
			name: "covar_pop window",
			query: `
SELECT COVAR_POP(y, x) FROM
  UNNEST([STRUCT(1.0 AS y, 1.0 AS x),
      (2.0, 6.0),
      (9.0, 3.0),
      (2.0, 6.0),
      (9.0, 3.0)])
`,
			expectedRows: [][]interface{}{
				// TODO(goccy/go-zetasqlite#): Use population covariance instead of sample covariance
				//expected output: {-1.6800000000000002},
				{-2.1},
				
			},
		},