[FR] Algorithm for unique ID generation

**Is your feature request related to a problem? Please describe.**
When there is a time we rely on firebase to generate unique ID for firestore or authentication, sometimes we don't want complete random ID. There should be some option to control the ID generation algorithm

I wish I could generate ID that can be always sorted. At least incremental or any monotonic function should be allow to prepend to the generated ID, such as base64 encoded unix timestamp

Additionally it should allow specifying the length of id, and the allowed encode character such as, is it case-sensitive, only numeric or alphanumeric, is it allow symbol, so on. So we have

**Describe the solution you'd like**
For authentication, maybe it could have option at the firebase console to specify the detailed algorithm as checkbox/radio

For firestore. Actually I wish I could have ability to do servervalue string formatting. So I could have firestore server generate any field, include ID, with the string I want

For this particular case I think we should have

```js
db.collection("users").add({ // Should allow push new document with __name__ as id
"__name__": ServerValue.calculate("string(request.time.toMillis(),16).reverse() + hashing.crc32(request.path).toHexString()"), // use firebase rule syntax for server generate value
data : "anydata"
});
```

The above example generate reverse hex string from time and hash of the path. This function guaranteed incremental from server timestamp but also seemingly random at start and the end

(Also I want to request that `string()` function in firebase rule system should have base argument for int value, and also reverse function)

**Describe alternatives you've considered**

There is a request to allow set custom ID in `beforeUserCreated` as requested in https://github.com/firebase/firebase-tools/issues/5675

Same go for firestore blocking function https://github.com/firebase/firebase-admin-node/issues/2077

**Additional context**
Is it possible to have firebase server issue request in github too?

从 Firestore 添加示例开始，审查关于在 beforeUserCreated 和 Firestore 阻塞函数中使用自定义 ID 的相关请求。明确范围涵盖 Authentication、Firestore 还是两者，然后在实现之前定义有序 ID、长度和字符规则以及服务器生成字段的验收标准。

由索引模型根据 Issue 内容生成。