HIP-21 Network info query

[steven-sheehy]

Problem

HIP-21 details a free network info query be added to the mirror nodes for use by the SDKs. We should target initially adding a gRPC API to the mirror node since SDKs don't currently support connecting to the REST API.

Solution

See the abandoned services proto PR for inspiration.

• Create a design document
• Create a network_service.proto both in mirror node repository and hedera-protobufs repository

message AddressBookQuery {
    FileID file_id = 1; // The ID of the address book file on the network. Can be either 0.0.101 or 0.0.102.
    int32 limit = 2; // The maximum number of node addresses to receive before stopping. If not set or set to zero it will return all node addresses in the database.
}

service NetworkService {
    rpc getNodes (AddressBookQuery) returns (stream NodeAddress);
}

• Create a NetworkController that implements the proto
• Create a NetworkServiceImpl with caching
• Create an AddressBookRepository that finds the latest address book
• Create an AddressBookEntryRepository

select * from address_book_entry
where node_id >= ? and consensus_timestamp = ?
order by node_id asc
limit 10;

Alternatives

Rejected since returning the full address book in one response would be too large. Current address book is 25K and growing.

/**
 * Response to a getNetworkInfo query.
 */
message NetworkInfoResponse {
    /**
     * The network <i>address book</i>, which includes not just the IP addresses of the network 
     * nodes, but also the the cryptographic keys they use to sign transactions and encrypt connections. Please 
     * see the <a href="https://hashgraph.github.io/hedera-protobufs/#proto.NodeAddress"><tt>NodeAddress</tt></a>
     * type for more details.
     */
    NodeAddressBook node_address_book = 1;
}

/**
 * Queries associated with the network as a whole.
 */
service NetworkService {
    /**
     * Retrieves free metadata about the network, most importantly its <i>address book</i>. 
     */
    rpc getNetworkInfo () returns (NetworkInfoResponse);
}