Add Node Agent Type to 'Filecoin.Version'

[rjan90]

Done Criteria

Add a new field to the Filecoin.Version JSON-RPC response that identifies the node's agent type (Lotus, Venus, Forest).

Why Important

Currently, there's no reliable way to determine which Filecoin implementation (Lotus, Venus, Forest) a node is running through JSON-RPC calls. This information can be useful for:

• Debugging and support scenarios where implementation details matter
• Monitoring and analytics systems to track network composition

Notes

Suggested by Hubert in Slack here