"Spec" for Service Provider Registry

## Done Criteria

There is a "spec" that defines the Service Provider Registry, including:
- Canonical provider metadata fields (`serviceProvider`, `payee`, `name`, `description`, `isActive`).
- Product metadata schema for SP offerings (starting with PDP), including required and optional capability keys ("known keys").
- Provider/product lifecycle semantics (register, update, remove, active/inactive behavior).
- Query and event expectations needed by integrators/indexers to consume registry state consistently.
- Extensibility/versioning guidance for adding future product types and capability keys without breaking compatibility.

## Why Important
- Aligns the SP Registry with standard Filecoin processes and gives implementers one official, stable spec.
- It reduces ambiguity across contracts, indexers, and client integrations.

## Notes
- We expect this to follow the pattern established in https://github.com/FilOzone/filecoin-services/issues/363