feat: Add ProxyStarted webhook event for dynamic port notification

[sripriya3191]

Summary

Add a new ProxyStarted server plugin webhook event that fires after a proxy has been successfully started and port allocation is complete.

Problem

The existing NewProxy webhook event fires before port allocation occurs. This means:

• When using remotePort = 0 for dynamic port allocation, the webhook only receives remote_port: 0
• External systems cannot know the actual allocated port via webhooks
• Users must poll the admin API or wait for NewUserConn events to discover the negotiated port

Solution

Add a new ProxyStarted webhook event that:

• Fires immediately after successful proxy registration and port allocation
• Includes remote_addr with the actual allocated port (e.g., :10000)
• Is a notification-only event (fire and forget) - plugins cannot reject since the proxy is already running
• Follows the same pattern as CloseProxy for consistency

Use Cases

1. Service Discovery: Register dynamically allocated ports with service discovery systems (Consul, etcd, etc.)
2. DNS Updates: Automatically update DNS records when proxies start
3. Monitoring: Track proxy lifecycle with actual port information
4. Audit Logging: Complete audit trail including negotiated ports

Changes

• pkg/plugin/server/types.go - Add ProxyStartedContent type
• pkg/plugin/server/plugin.go - Add OpProxyStarted constant
• pkg/plugin/server/manager.go - Add proxyStartedPlugins slice and ProxyStarted() method
• pkg/config/v1/validation/validation.go - Add ProxyStarted to SupportedHTTPPluginOps
• server/control.go - Call ProxyStarted webhook after successful proxy registration
• doc/server_plugin.md - Update documentation

Configuration

# frps.toml
[[httpPlugins]]
name = "port-notifier"
addr = "127.0.0.1:9000"
path = "/handler"
ops = ["ProxyStarted"]

Webhook Payload

{
    "version": "0.1.0",
    "op": "ProxyStarted",
    "content": {
        "user": {
            "user": "myuser",
            "metas": {"key": "value"},
            "run_id": "abc123"
        },
        "proxy_name": "my-tcp-proxy",
        "proxy_type": "tcp",
        "remote_addr": ":10000"
    }
}

Comparison: NewProxy vs ProxyStarted

Event	When Fired	remote_port / remote_addr	Can Reject?
NewProxy	Before port allocation	Requested port (may be 0)	Yes
ProxyStarted	After port allocation	Actual allocated port	No

Backward Compatibility

This is a purely additive change:

• No existing behavior is modified
• New event is opt-in via plugin configuration
• Existing plugins continue to work unchanged

[fatedier]

We’re not going to accept this PR.

The actual assigned port/address can already be retrieved via the frps API.

[sripriya3191]

Thank you for the suggestion to use the frps API. While that's a valid approach, I really like the existing server-side plugin/hook functionality and thought this would be a natural extension to allow external services to be notified of allocated ports in real time. Let us know your thoughts!