Disable login prompt

**Is your feature request related to a problem? Please describe.**
Every time I [re]launch vscode or reload a window I am prompted to "Sign in to GitHub to access your repositories and GitHub Actions workflows."

**Describe the solution you'd like**
Add an option to disable this prompt.

**Additional context**
Shortly prior to the official adoption of this extension, progress was made on the original repo to allow the relevant parts of the extension (syntax highlighting, public action tooltips, etc) to function without logging in. That left it in the current state, with those features working, but the prompt still always popping up.

Start by locating the VS Code extension's login prompt and the existing paths that support syntax highlighting and public action tooltips without authentication. Determine where an extension option can control the prompt, then verify that disabling it suppresses repeated sign-in requests while those unauthenticated features continue to work.

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