Ability to name the app for usage

[guptaeshan95-stack]

Summary
Enable users to configure or override the client application name (e.g., openclaw, codex, etc.) when making requests through Manifest to external providers.

Problem
Some model providers (e.g., Kimi coding plan) restrict access based on the client application name or identifier. Currently, Manifest sends its own default identifier, which results in compatibility issues or outright rejections—even when the actual usage aligns with supported applications like OpenClaw.

Example Issue
While attempting to use the Kimi coding plan via Manifest, requests fail with an error indicating that access is limited to specific applications such as codex or openclaw. Although the functional use case matches OpenClaw (coding assistance), the request is rejected due to the client identifier mismatch.

Proposed Solution
Introduce a configurable option in Manifest to:
Override the client/app name sent in API requests