[C++] Right-justify array values in PrettyPrint

Currently we the output of PrettyPrint for an array looks as follows:

[
  1,
  NA
]

We should right-justify it for better readability:

[
   1,
  NA
]

Reporter: Uwe Korn / @xhochy

Related issues:

• PrettyPrint Improvements (is a child of)

Note: This issue was originally created as ARROW-2885. Please see the migration documentation for further details.