Add RFC for XTable REST Service

[rahil-c]

What is the purpose of the pull request

• Adds an RFC for an external REST Service that will run Apache XTable for metadata conversions.
• The following google docs captures the initial design, as well as the community's feedback: https://docs.google.com/document/d/13ADN-TuuMd8626y3jJUVL3EMh1X3sbQWAP2p8e7PZ9I/edit?tab=t.0#heading=h.wugd9y1127zk

Brief change log

• Adds an RFC which discusses the design considerations for this new service.

Verify this pull request

• • To see the full rest spec see the following pr: Add open api REST spec for XTable Service #705

[rahil-c]

cc @vinishjail97 @the-other-tim-brown