Handle chat creation and invitations

# Is your feature request related to a problem?

A user can subscribe to notifications related to a smart contract address.
He will then receive a PUSH notif and/or a Websocket notification every time a user emits a transaction and asks backend to notify.

Although this works well when you already know the addresses of the discussion smart contracts, it does not cover invitations to new conversations.

We need add a subscription to get notifications when we are invited to a new Chat.

# Describe the solution you'd like

## New chat publication

### Request
After creating a new chat, the creator must send the following request.

```ts
POST
/chatCreated

body: {
smartContractGenesisAddress: string // Chat smart contract address
signedPayload: string // privKey.sign(smartContractGenesisAddress)
}
```

### Backend validation :

Before sending notifications, the backend must ensure the request is legitimate :

consider `pubKey = Tx.read(smartContractGenesisAddress).previousPublicKey)`

- `pubKey.verify(signedPayload, smartContractGenesisAddress)`
- `Tx.read(smartContractGenesisAddress)` created by `pubKey`
- `Tx.read(smartContractGenesisAddress)` created less than 1 min ago
- No notification sent yet for `smartContractGenesisAddress` creation

### Notifications send

Backend will then read the smart contract to extract the chat members `pubKey`s.

Notification will be sent to all the members.

## Message send

This is a rework of the `transactionSent` endpoint.

It mainly adds a validation to ensure the transaction is a message of the chat smart contract.

### Request

Rename the endpoint `transactionSent` to `messageSent`.

```ts
POST
/messageSent

body: {
pubKey: string // Message creator pubKey
messageAddress: string // Message Tx address
smartContractGenesisAddress: string // Chat smart contract address
signedPayload: string // privKey.sign(messageAddress)
}
```

### Backend validation :

Before sending notifications, the backend must ensure the request is legitimate :

consider `pubKey = Tx.read(smartContractGenesisAddress).previousPublicKey)`

- `Tx.read(messageAddress)` is a `call` `input` of `Tx.read(smartContractGenesisAddress)` // Ensures the message is related to this chat
- `pubKey.verify(signedPayload, messageAddress)`
- `Tx.read(messageAddress)` created by `pubKey`
- `Tx.read(messageAddress)` created less than 1 min ago
- No notification sent yet for `messageAddress` creation

### Notifications send

Backend will then check :
- all the clients subscribed to `smartContractGenesisAddress`
- all the members of the chat smart contract

Backend will notify members of both sets.

Start at the existing transactionSent endpoint and trace its notification flow. Define the new chatCreated and messageSent entry points, then verify the stated signature, transaction-age, chat-membership, and duplicate-notification validations. Done means newly created chats notify their members, messages notify subscribers and chat members, and transactionSent is replaced by messageSent.

Written by the indexing model from the issue text.