Add lower() to string operations

[arwentwinkle]

System information

• TensorFlow version (you are using): 1.10 and 1.12
• Are you willing to contribute it (Yes/No): No

Describe the feature and the current behavior/state.
Add the string lower() function to the tf.string class.
Current state has no way to lower case input strings. This requires it to be done at dataset build time which reduces dataset flexibility

Will this change the current api? How?
Would add extra functionality

Who will benefit with this feature?
All users doing natural language processing

Any Other info.

[yongtang]

Added a PR #25859 with strings.lower() and strings.upper().