Feat(agenthub): Add new agent, ProxyAgent to enable collaboration among multiple OpenHands instances

[f-miyake]

This pull request introduces a new agent named Proxy Agent that extends the current delegation mechanism. In addition to handling local task delegation, the Proxy Agent is designed to delegate tasks to agents hosted on external OpenHands instances. This enhancement is a step toward enabling our system to integrate with external AI agent platforms.

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 PR introduces the Proxy Agent, a new agent that extends the existing task delegation mechanism by enabling delegation to remote agents hosted on separate OpenHands instances.
• The Proxy Agent works with function calling and can choose from two tools, DelegateLocalTool for the existing local delegation and DelegateRemoteOHTool for the remote delegation to an agent on a different instance of OpenHands.
• The DelegateRemoteOHTool is responsible for sending messages to agents running on different OpenHands instances. This tool is part of a separate feature introduced in feat(agent_skills): Add OpenhandsClient to agent_skills to enable collaboration among multiple OpenHands instances #6492 (not merged yet), which enhances OpenHands' ability to communicate with external agents.
• The list of available agents, both local and remote, is defined in openhands/agenthub/proxy_agent/agent_list.json.
• For more details, please refer to the README.md in openhands/agenthub/proxy_agent directory.

[neubig]

I will take a look at this! Apologies that I have not fully checked the old one yet.