Adding an API to get light client info

[Keith-CY]

This issue is a feature request of adding an API to get light client info, similar to get_node_info of CKB node and get_indexer_info of CKB indexer.

Neuron is going to integrate with the light client. However, the network(endpoint) connected by neuron is set by users, and the network cound be either a CKB node, or a light client. So detecting the service of an endpoint is required.

Once the API is ready, program can check if it's a CKB node or a light client without extra info provided by users, and neuron will be network-agnostic

Ref:

• https://github.com/nervosnetwork/ckb/tree/develop/rpc#method-local_node_info
• https://github.com/nervosnetwork/ckb-indexer#get_indexer_info