feat: add PingNode RPC protobuf definitions

[roganlynch]

Adds PingNode RPC protobuf definitions for the gRPC API.

Changes

• PingNodeRequest: Protobuf message definition for ping requests
• PingNodeResponse: Protobuf message definition for ping responses
• RPC service definition: Adds PingNode method to the HeadscaleService
• Generated code: Includes compiled protobuf Go code

Implementation Details

Defines the gRPC contract for the PingNode API, allowing clients to request node pings through the headscale control API. The request includes node identification, and the response provides ping status and results.

Context

Part of the work discussed in #2902 to add ping functionality to headscale.

Dependencies

• Requires PR #2940 (ping scheduler)

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Note: This code was generated with assistance from Claude Sonnet 4.5 via cline.bot