"Spec" for data set and piece metadata

## Done Criteria
There is a "spec" that defines the metadata of data sets and pieces including "known keys".

## User
builders using FOC or maintainers in the future (i.e., what information do I wish I had documented so I could be self service in building or maintaining)

## Why Important
It serves as "official" documentation.

## Notes
1. We expect this to live in a foc specs repo.
2. We're actually thinking not to make this an FRC because: - that template isn't the most useful
- we want to lean into being very agent friendly
- more much friction and formal rules and formats,
2. We expect to have something as a living reference and maybe FRCs can pop out of that but only as a token to throw towards the FIPs repo if we think that the connectivity with the broader ecosystem requires it. We hope that we can just bridge "foc specs" and FIPs naturally: FOC stuff lives over here and is described in this way, core protocol stuff lives over there and is described in that way...

## Related items
https://github.com/FilOzone/filecoin-services/issues/434
https://github.com/FilOzone/filecoin-services/issues/455