internal/ethapi: add net_nodeInfo

[weiihann]

Description

This PR adds a new net_nodeInfo RPC method. It is the same as admin_nodeInfo.

Rationale

The reason why net_nodeInfo is introduced even though admin_nodeInfo already exists is because some admin methods can control the behavior of the node. If a node were to be compromised with the admin namespace enabled, then the node could potentially stop functioning.

On the other hand, having access to retrieving the node information is extremely useful, especially when running an infrastructure. The content of the node info doesn't reveal sensitive information as well, so it could be added to the net namespace safely.

Another consideration is to remove admin_nodeInfo and replace with net_nodeInfo. This is possible, but this will contradict with the documentation of geth upstream. So in my opinion, there's no harm keeping both.

Example

curl -H "Content-Type: application/json" -X POST --data '{"jsonrpc":"2.0","method":"net_nodeInfo","params":[],"id":83}' 127.0.0.1:8575