[FR] Blocking function for firestore document modification at create/update/delete

**Is your feature request related to a problem? Please describe.**
There are many situation that we just not only want to protect document from being written (which could be done by security rules), but to also inject data into document or modified something complicate or written data by some logic on the server

For example, user might modified some field about their profile, such as their name or email. And then we decide that we want to index all their named profile into array of lowercase text chunks. So we could do text search with firestore `in` query

But utilizing `onWrite` function to written complicate checking logic and update same document 2 times every time user change their profile is not ideal. Especially it was admittedly risking infinite loop

**Describe the solution you'd like**
Taken idea from firebase auth blocking function, firestore should also have blocking function, `beforeWrite` function that provide the same parameter as `onWrite` function but need to return document, or throw exception to cancel writing document

Taken example from https://firebase.google.com/docs/firestore/extend-with-functions#reading_and_writing_data

```js
// Listen for updates to any `user` document.
exports.countNameChanges = functions.firestore.document('users/{userId}').onUpdate((change, context) => {
// Retrieve the current and previous value
const data = change.after.data();
const previousData = change.before.data();

// We'll only update if the name has changed.
// This is crucial to prevent infinite loops.
if (data.name == previousData.name) {
return null;
}

// Retrieve the current count of name changes
let count = data.name_change_count;
if (!count) {
count = 0;
}

// Then return a promise of a set operation to update the count
return change.after.ref.set({
name_change_count: count + 1
}, {merge: true});
});
```

Could be changed into

```js
// Listen for updates to any `user` document.
exports.countNameChanges = functions.firestore.document('users/{userId}').beforeWrite((change, context) => {
// Retrieve the current and previous value
const data = change.after.data();
const previousData = change.before.data();

if (data.name != previousData.name) {
data.name_change_count = (previousData.name_change_count || 0) + 1;
}

data.lastUpdate = FieldValue.serverTimestamp(); // now we can also index updateTime without exposing metadata updateTime
// Must return the data
return data;
});
```

**Describe alternatives you've considered**
Alternatively this could be done by the security rule directly if we have security rule v3 with more syntax to allow set feature

**Additional context**
I have not sure if it (and should it) allow blocking function in firestore with async function and transaction. Maybe it should be sync function but can request for dependency document

```js
// Listen for updates to any `user` document.
exports.countNameChanges = functions.firestore.document('users/{userId}')
.requireDocs((userId) => ["userAdditionalProfiles/" + userId,"admins/" + userId]) // array of `collection/docId` to request some documents from other collections
.beforeWrite((change, context,[profile,admin]) => {
// Retrieve the current and previous value
const data = change.after.data();
const previousData = change.before.data();

if (data.name != previousData.name) {
data.name_change_count = (previousData.name_change_count || 0) + 1;
}

if(admin.exist && data.adminLevel != admin.get("level")) {
data.adminLevel = admin.get("level"); // always sync user with admin level for indexing and querying
}

// Must return the data in sync
return data;
});
```

Or maybe providing transaction to write multiple document too?

この issue では、Firestore ドキュメントトリガー、onWrite API と提案されている beforeWrite API、および Firebase Functions のドキュメントについて述べられていますが、リポジトリのファイルやテストはありません。まず既存の Firestore トリガーのエントリポイントとリンク先のドキュメントを確認し、次に create/update/delete、非同期処理、トランザクション、キャンセルのサポートを明確にします。API の範囲と受け入れ基準について合意できれば完了です。

索引モデルが issue の本文から書いたものです。