Client Side State Encryption

Issue: https://github.com/opentofu/opentofu/issues/874

This RFC document exists to point to the original GitHub issue RFC which grew in size/scope/complexity to the point that it was unintelligible. @JanosDebugs, @StephanHCB, @Yantrio, and @cam72cam got together and created state_encryption.md to refine and converge on a single understanding of the user experience and the work to be done.

This is historically significant as it was a clear breakdown of the RFC process and exposed some serious limitations with the OpenTofu RFC workflow. The understanding gained by the transition from GitHub issue to live document revised by PR formed the basis of the new RFC process introduced in OpenTofu RFC Process.