Add `flatten()` builtin

[antonmedv]

Add flatten builtin.

flatten([["a", "b"], [1, 2]]) == ["a", "b", 1, 2]

[OlgaNovg]

flatten([["a", "b"], [1, 2, [3, 4]]]) == ["a", "b", 1, 2, 3, 4] or ["a", "b", 1, 2, [3, 4]] ?

[antonmedv]

["a", "b", 1, 2, 3, 4] Lets make it to all nested arrays.

[vmihailenco]

Would be nice if the function also supports maps, e.g. just like VRL https://vector.dev/docs/reference/vrl/functions/#flatten