Ip based allow list for readonly access

[johansolve]

Feature request: ip based allow list for readonly access.

Background: To be able to get n2k data via Signal K api into OpenCPN on port 3000, public readonly access must be enabled. But this exposes very much on the web interface of Signal K without logging in, which is not desirable when Signal K web interface is publicly accessible. Preferably unauthenticated access should only be allowed from localhost or designated IP. From other clients authentication should be required.