Alter carbon schema related to complex columns

Add complex columns(only array and struct) to carbon table. For example - 

array<int>, struct<a:int, b:string>

Command - 

ALTER TABLE <table_name> ADD COLUMNS(arr array<double>)

Design document - https://docs.google.com/document/d/1DhhkVXM8rMvOuKDZeccJpFEfO3VkA9C0c7JHCV88NXI/edit