3rd party GitHub App installation on enterprise accounts [Public Preview]

### Value Prop
Enterprise admins can now install third-party GitHub Apps on their enterprise account, opening up a broader range of automation and management tools. This means SaaS app developers can now integrate with the enterprise account to help manage common enterprise workflows — such as identity management, member provisioning, and team organization — using a new or existing GitHub App and the enterprise permission set.

### Expected Outcome
Customers gain access to a wider set of third-party integrations for enterprise account management, reducing the need for custom-built solutions. As more software vendors add enterprise account support to their apps, admins will have an easier time automating common tasks like SCIM provisioning, member lookup, and metrics gathering.

The issue provides a product description but names no repository files, tests, or implementation entry points. Start by identifying where GitHub Enterprise account and GitHub App installation behavior is implemented. Done means third-party GitHub Apps can be installed on enterprise accounts with the described enterprise management support.

索引モデルが issue の本文から書いたものです。