Portal: Split responsibilities

[Karrq]

The Portal is a critical component in the stack, sitting in between the consensus client (OP Node) and the EL, forwarding calls to the current gateway and a fallback geth node.

Currently the Portal also acts as a proxy for various namespaces and also as a proxy for the registry, along with handling gateway selection.

To address this, we could:

1. Isolate the Registry and gatewasy selection logic in a standalone service (possibly public)
2. Remove the RPC proxy functionality entirely: users would interact with the gateways directly instead of thru the portal

If the RPC proxy is still desirable we could have a dedicated service instead, altough this would move the point of failure to this proxy

[Karrq]

Registry already has own service, and Portal doesn't do much "public" proxying