Add array_median function

It is known that there isn't any exact median function in Spark SQL, and this might be a difficult problem to solve efficiently. However, to find the median for an array should be a simple task, and something that users can utilize when collecting numeric values to a list or set.

This can already be achieved by using sorting and choosing element, but can get cumbersome and if a fully tested function is provided in the API, I think it can solve some headache for many.