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.

该 issue 提供了产品描述，但没有指出任何 repository 文件、测试或实现入口。首先确定 GitHub Enterprise 账户和 GitHub App 安装行为的实现位置。完成的标准是：第三方 GitHub Apps 可以安装到具备所述 Enterprise 管理支持的 Enterprise 账户上。

由索引模型根据 Issue 内容生成。