avoid triggering preflight for every RPC call

[lidel]

(did not look into this too deep, filling issue so we dont forget to investigate at some point)

Right now (v4.3.0) in Chrome v128 every RPC call done cross origin (https://webui.ipfs.io/, https://bafybeiev6pghzkw7bnyt73sompnrszdwjfuvyzl5tg43v5bfwewqn7an4a.on.fleek.co/) triggers a preflight call (?):

We should see if we can avoid this, perhaps by adjusting cache control or Access-Control-Max-Age headers returned by Kubo.