feat(agent_skills): Add OpenhandsClient to agent_skills to enable collaboration among multiple OpenHands instances

[f-miyake]

This change adds the OpenhandsClient class and the message_to_remote_OH function to agent_skills, enabling the ability to send messages to other OpenHands instances.
The agent can make use of this tool via IPythonRunCellAction.

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions
This function is designed for agent developers to create agents that collaborate with OpenHands instances hosted on other servers. Currently, only sending Message Actions is supported, but I plan to add features such as file uploads in the future.

Additionally, I am planning to implement a Proxy Agent into agenthub that enables collaboration between OpenHands instances hosted separately by different users or organizations by utilizing this functionality.
I am also considering developing a client that can connect to agent platforms other than OpenHands, aiming to facilitate collaboration across different platforms.
The changes in this PR are intended to be the first step toward that.

flowchart LR
    subgraph Server1
        A["CodeActAgent"] -->|delegate| B["Proxy Agent<br> (To be implemented)"]
    end
    subgraph Server2
        C["Other Agent"]
    end
    B -->|message_to_remote_OH| C

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