[Go] implement Stringer for DataTypes

[asfimport]

Reporter: Sebastien Binet / @sbinet
Assignee: Sebastien Binet / @sbinet

PRs and other links:

• GitHub Pull Request #4161

_{Note: This issue was originally created as ARROW-5174. Please see the migration documentation for further details.}

[asfimport]

Sebastien Binet / @sbinet:
Issue resolved by pull request 4161
#4161