frpc: support metadatas and annotations in frpc proxy commands

[fatedier]

WHY

Fix #4440

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[morph027]

Wow, thank you 🫶

[morph027]

Just created a test build from dev and it looks like the metadatas are not being passed to the server.

Client:

frp/bin/frpc http -s <server> -d <domain> --user <user> --metadatas token=<token>
2025-01-06 18:30:55.728 [I] [client/service.go:295] try to connect to server...
2025-01-06 18:30:55.811 [E] [client/service.go:280] send Login request to plugin error
2025-01-06 18:30:55.811 [W] [client/service.go:298] connect to server error: send Login request to plugin error
login to the server failed: send Login request to plugin error. With loginFailExit enabled, no additional retries will be attempted

Server plugin shows the following payload and therefore is unable to validate the token.

{
  "version": "0.61.1",
  "os": "linux",
  "arch": "amd64",
  "user": "<user>",
  "privilege_key": "3a108ca101d3bea67f6892ec6ee7e82a",
  "timestamp": 1736184655,
  "client_spec": {},
  "pool_count": 1,
  "client_address": "<client-ip:client-port>"
}

[fatedier]

It's metadata for proxy, not for client.

Command-line arguments are not very convenient for expressing complex hierarchical structures, so supporting client metadata may not be considered.