Implement flatten function

[izveigor]

Is your feature request related to a problem or challenge?

Summary

Characteristic	Description
Function name:	flatten
Aliases:	-
Original function?:	No
Function Description:	Azure DataBricks: Returns an array of the elements in the union of array1 and array2 without duplicates. Spark SQL: Creates a single array from an array of arrays column. ClickHouse: Converts an array of arrays to a flat array. Function: Applies to any depth of nested arrays. Does not change arrays that are already flat. The flattened array contains all the elements from all source arrays. DuckDB: Concatenate a list of lists into a single list. This only flattens one level of the list (see examples).
Sources:	Concept Azure DuckDB ClickHouse Spark

Examples:

D select flatten([[1, 2, 3], [4, 5, 6]]);
┌──────────────────────────────────────────────────────────────────────────────┐
│ flatten(main.list_value(main.list_value(1, 2, 3), main.list_value(4, 5, 6))) │
│                                   int32[]                                    │
├──────────────────────────────────────────────────────────────────────────────┤
│ [1, 2, 3, 4, 5, 6]                                                           │
└──────────────────────────────────────────────────────────────────────────────┘
D select flatten([[[1, 1], [2, 2], [3, 3]], [[4, 4], [5, 5], [6, 6]]]);
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ flatten(main.list_value(main.list_value(main.list_value(1, 1), main.list_value(2, 2), main.list_value(3, 3)), main.list_value(main.list_value(4, 4), main.li…  │
│                                                                           int32[][]                                                                            │
├────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
│ [[1, 1], [2, 2], [3, 3], [4, 4], [5, 5], [6, 6]]                                                                                                               │
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

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[jayzhan211]

Let me take a look on this issue.

[izveigor]

That sounds good, @jayzhan211! I only want to warn you that DuckDB version of flatten function is different from the rest (decreases only 1 level than the rest to 1 dimensional array). I think the last option is much better (ClickHouse, Spark and Azure DataBricks).

[jayzhan211]

For the latter version, it seems the function is implemented in #6796. I will continue on this after that PR merged.