Add file action result streaming support to RPC datasource and agent

• RpcActionRoute.handleExecute in the rpc-agent now detects File action results and streams the file body with Content-Disposition, X-Forest-Action-Type, and X-Forest-Action-File-Name headers; optional responseHeaders are forwarded via X-Forest-Action-Response-Headers.
• RpcCollection.execute in the datasource-rpc now bypasses superagent's JSON parsing, buffers the raw response, and inspects X-Forest-Action-Type to reconstruct a File ActionResult (with a Readable stream, mimeType, and decoded filename) or falls back to manual JSON parsing for other result types.
• Filenames are URL-encoded on the agent side and decoded on the datasource side to handle special characters safely.

Changes since #217 opened

• Refactored variable naming and formatting in RpcCollection.performAction method [6efcb19]

^{Macroscope summarized bb46f93.}