Add method to get the installation ID

### Problem Statement

In our App we have a button which allows the user to report issues.
For that, we would love to add some optional information like the installation ID from Sentry, so we can use that to analyse the user issues.
Right now I have no way to consistently get the information.

I tried setting a User manually, without any ID, which should lead to using the installation ID, which I should then be able to read out from the current scope, but that is not working reliably.

### Solution Brainstorm

So what I would love to have:

Sentry.getInstallationId() which would return the installation DI or Sentry.getUser(), which would return a default User, with the installation ID as user id.

先閱讀 Sentry SDK 中現有的 User 與 current-scope 處理方式，接著追蹤 installation ID 的建立與持久化方式。定義公開 API 應直接公開該 ID，還是透過預設的 User 公開，並確認回傳值在相關的 SDK 生命週期中保持一致。

由索引模型根據 Issue 內容生成。