[webchannel] Add GitHub Actions workflow to deploy message generator demo

[sampajano]

Description

This PR adds a GitHub Actions workflow (.github/workflows/webchannel-deploy-message-generator.yml) to automatically build and deploy the TypeScript WebChannel Message Generator demo (webchannel/js/demo) to GitHub Pages under the dedicated /webchannel/message_generator/ subpath upon pushing to the main branch.

Architecture & Pipeline Overview

Destination URL (Post-Merge)

• Message Generator Demo: https://google.github.io/net_http/webchannel/message_generator/

Key Details

• Renamed Source File: Renamed demo.ts to message_generator.ts which compiles to dist/message_generator.js.
• Subpath Organization: Stages the demo exclusively under _site/webchannel/message_generator/ as the Pages artifact.
• Trigger: Automatic on push to main branch, with support for manual triggers via workflow_dispatch.
• Security Compliance: All GitHub Action references are pinned to full commit SHAs complying with zizmor security checks.

Verification & Live Preview

• Fork Test URL: https://sampajano.github.io/net_http/
• Successfully tested on fork repository with end-to-end GitHub Pages build and deployment.

[sampajano]

@wenbozhu I configured it so the demo url would be the following (it will work after PR is merged):

https://google.github.io/net_http/webchannel/

[wenbozhu]

url path: /net_http/webchannel/message_generator
js/ts file: s/demo/message_generator/
file path: _site/webchannel/demo/message_generator.js|ts

feel free to suggest a shorter name for message_generator

[sampajano]

url path: /net_http/webchannel/message_generator js/ts file: s/demo/message_generator/ file path: _site/webchannel/demo/message_generator.js|ts

feel free to suggest a shorter name for message_generator

@wenbozhu Done. Renamed everything to message_generator now. PTAL :)