Add logo_url to GitHub App manifest / Add PATCH /app endpoint

[elitan]

When creating GitHub Apps via the manifest flow, there is no way to programmatically set the app's logo/avatar.

The manifest supports name, description, url, permissions, events, etc. - but no branding.

Use case

Platforms that let users create their own GitHub App (via manifest flow) for self-hosted integrations. We want the app to display our platform's logo instead of a generic identicon.

Proposed solutions (either would work)

1. Add logo_url parameter to the manifest spec - GitHub fetches and sets the image during app creation
2. Add PATCH /app endpoint to update app settings (including logo) after creation

Currently the only way to set a logo is manually through the web UI, which breaks the automated flow.

[typed-sigterm]

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