[Python] Store sequences of arbitrary ndarrays (with same type) in Tensor value type

[asfimport]

This can be implemented using extension types, based on outcome of ARROW-1614

Reporter: Wes McKinney / @wesm
Watchers: Rok Mihevc / @rok

Related issues:

• [C++] Add a Tensor logical value type with constant shape, implemented using ExtensionType (relates to)

_{Note: This issue was originally created as ARROW-5819. Please see the migration documentation for further details.}