docs(rfc): clarify shared-supervisor workload identity in RFC 0018 #4649

Problem

Section 5 of RFC 0018 in #4377 needs to incorporate the workload-identity review feedback before the shared-supervisor design is finalized. It currently assumes a sandbox-side gRPC client and proactive SVID forwarding, and leaves gateway subject binding and trusted middleware use underspecified.

Scope

Revise section 5 only. This is an RFC documentation task; it does not implement identity acquisition or change dedicated-supervisor behavior. The contribution targets the RFC owner's branch and is part of #4311.

Acceptance criteria

• Describe on-demand, audience-specific acquisition through a trusted interface, with sandbox/run/ownership binding and bounded caches.
• Require the gateway to independently match the SVID subject to the authenticated sandbox.
• Describe authorized middleware injection of a JWT-SVID into a destination-bound custom header.
• Explain that Kata's host-to-guest UDS limitation is independent of supervisor topology; identify a VM-local Teleport bot socket as a provider-specific option requiring validation and isolation from agent processes.
• Preserve per-sandbox identity, fail-closed behavior, and private-key confinement.

Review threads: acquisition and Kata, gateway binding, on-demand acquisition.