Expose routing system under /routing/v1

[lidel]

Checklist

• My issue is specific & actionable.
• I am not suggesting a protocol enhancement.
• I have searched on the issue tracker for my issue.

Description

In effort to enable self-hosted deployments, and reduce IPFS ecosystem dependency on cid.contact for DHT proxying purposes, we should expose preexisting routing system from Kubo via /routing/v1 (introduced in IPIP-377) on the same HTTP port we use for /ipns and /ipfs Gateways.

Why

• Helia users need to be able to prototype and self-host without reliance on a third-party infrastructure
• Routing system is already present in Kubo, and we already expose it over RPC, which makes this is only a "cosmetic" change to expose it using alternative, vendor-agnostic API IPFS community agreed on, and dogfood boxo/http/routing/server library

What

• add Gateway.ExposeRoutingAPI (Flag, disabled by default for now, we can flip the switch in the future)
• make it expose /routing/v1 from IPIP-337 (application/json), equivalent of ipfs routing findprovs
• add streaming support (application/x-ndjson) (IPIP to be written, code in feat: update boxo with routing streaming #9868 and tests wip Missing tests for streaming /routing/v1 #9873)
• (TBD) IPNS support (IPIP-351: IPNS Signed Records Response Format on HTTP Gateways  specs#351)
• (TBD) Peer routing support (IPIP needed)