Allow optional use of field labels

[omkar-dsd]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

Expected behavior

Allow labels to be specified optionally which is already a property of field. If user wants to return the field labels in place of field name, he/she should be able to configure so by setting it up in the extra_kwargs

Actual behavior

Currently, there is no such option to provide label in to_representation

[rpkilby]

Closing, per #6649