[Tracking issue] Start flownode as a service

[discord9]

What type of enhancement is this?

Feature

What does the enhancement do?

Run the flownode as a stateless and scalable service, similar to a datanode:

greptime flownode start

(or maybe flownode will be a better naming that is consistent with datanode).

Tasks

The following are the key tasks for implementing the feature. We can still add other related PRs.

1. Add the control plane between flownode and metasrv (@discord9 @fengjiachun)

   Similar to datanode, the flownode service needs to interact with the metasrv.

   • feat: enable metasrv to accept flownode's heartbeats #4160 @fengjiachun
   • Register flownode to metasrv; feat(flow): flow node addr key #4164 feat: register flow node #4166
   • Send a heartbeat from flownode to metasrv(similar to datanode); @discord9 feat: heartbeat task&peer lookup in proc #4179

2. Add the gRPC service for flownode (@zyy17)

   It may depend on 1.

   • Add the gRPC service for flownode;

3. Add the subcommand for flownode (@zyy17)

   It depends on 1 and 2; maybe it can be part of 2.