Added API for Redis set operations

[rob-deans]

Description

Motivation and Context

How Has This Been Tested?

Made tests for both the String version of the function and the RedisString

Checklist:

• [ ] I have submitted a CLA form
• [X] If applicable, I have updated the documentation accordingly.
• [X] If applicable, I have added tests to cover my changes.

[rob-deans]

@shmuelk Can you please review this again?