[SPARK-2096][SQL] support dot notation on arbitrarily nested array of struct

The rule is simple: If you want `a.b` work, then `a` must be some level of nested array of struct(level 0 means just a StructType). And the result of `a.b` is same level of nested array of b-type.
An optimization is: the resolve chain looks like `Attribute -> GetItem -> GetField -> GetField ...`, so we could transmit the nested array information between `GetItem` and `GetField` to avoid repeated computation of `innerDataType` and `containsNullList` of that nested array.
@marmbrus Could you take a look?
use `def` or `lazy val`?