"Spec" for Session Key Registry

## Done Criteria
There is a story that outlines the structure, intent, and limitation of the Session Key Registry.

## Why Important
Provide public documentation on this generic building block in FOC.

## Notes
1. This is the Session Key Registry equivalent of https://github.com/FilOzone/filecoin-services/issues/434
2. We expect this to follow the pattern established in https://github.com/FilOzone/filecoin-services/issues/363