rfcs: add RFC-20 to extend tenants to multicast use cases

[juan-malbeclabs]

Summary

• Adds RFC-20, proposing the extension of Tenant from a unicast-only construct to a per-use-case resource grouping primitive.
• Tenants declare which traffic classes they participate in (unicast, multicast, or both) and only allocate a VRF ID when they need one.
• Multicast groups gain an optional tenant_pk on create and update; mandatory enforcement comes in a follow-up RFC.
• Tenant administrators can manage their own access passes without going through foundation, sentinel, or feed authority.

Why

• Every tenant today consumes one of the 1024 global VRF IDs, even when it exists only to group multicast resources. The pool becomes scarce for tenants that actually carry unicast traffic.
• Access passes require foundation/sentinel/feed authority, which blocks self-service onboarding for the team running a use case.
• One consistent construct per use case is cleaner than introducing a separate grouping primitive for multicast.

Testing Verification

RFC document only; no implementation in this PR. Implementation lands in follow-up PRs scoped to the smartcontract, SDKs, and CLI.

RFC-20