specs/xrpc: short section on service proxying header

[bnewbold]

Spec is pretty simple so far. I hedged things a bit with a disclaimer, as I won't be surprised if we end up adding:

• /.well-known/ pre-flight check
• ability to not forward on service token (or reduce it's scope)
• details about what other headers should be passed along (or not), including content negotiation
• whether to do a "forwarded for" header or not (probably not, for privacy reasons?)
• caching
• websockets
• server-side retries, timeouts, circuit-breaking
• status codes

This references the labels spec, which hasn't merged yet.

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