Send session started events to the API

[aleks-tpom6oh]

Client should group event schemas into sessions.

Session is a rolling 5 minutes window. If an event schema is send before 5 minutes from the last event schema passed - it's the same session.
If an event is sent after 5 minutes since previous event - it's a new session.

Also, if it is a first event schema sent from current app instance - it's a new session.

Session is identified by a session id - a UUID.

API doc

Endpoint: api.avo.app/inspector/v1/track
Method: POST

Every time new session starts we need to send the following payload to the API:

{
  [
    {
      "appName": ...,
      "appVersion": ...,
      "libVersion": ...,
      "libPlatform": "flutter",
      "apiKey": ...,
      "sessionId": "11332f62-ba2d-11eb-8529-0242ac130003", // UUID
      "messageId": "4cffbdca-ba2e-11eb-8529-0242ac130003", // UUID
      "createdAt": "yyyy-mm-ddThh:mm:ss.SSSZ",
      "type": "sessionStarted",
    },
  ]
}

### HTTP Responses

- Server returns a `200` response for successful API requests.
- Server returns a `400` response for invalid requests.
Invalid requests include:
    - `Content-type header missing`
    - `Invalid API key

Write tests for the session tracking and the payload generated for the API calls.

Reference

https://github.com/avohq/node-avo-inspector/blob/main/src/AvoSessionTracker.ts
https://github.com/avohq/node-avo-inspector/blob/main/src/__tests__/Sessions.ts