Add Get Session and Async New Session

[christian-bromann]

This is the light version of #1552 that keeps implementation effort for browser vendors low while enabling intermediary nodes the ability to create async sessions in a lightweight way. There are two new endpoints being added in this patch:

1. Get Session
   Returns the capabilities of an active session. This is also very useful for endpoint nodes
   in the scenario where a clients wants to attach to an active session receive the capabilities
   it has attached to. This information is not necessarily given for client implementations.

2. Async New Session
   Endpoint nodes run the same session creation steps as in New session but return only the
   session id. Intermediary nodes keep a list of "session creation jobs" that maps "session
   creation job ids" to session ids of active sessions.

Now the use case is a user can now always make an async session by making a request to [POST] /session/async which returns a session creation id:

{
  "sessionCreationId": "<uuid>"
}

With this id the "Get Session" endpoint can be requested, which returns with unknown session as long as the intermediary node has not yet initiated the session at the endpoint node. Once that happen a response with the following data will be returned:

{
  "sessionId": "<uuid of active session>"
  "capabilities: { ... }
}

The client can then continue making WebDriver request with the session id it received from this response.

I am happy to start working on the web platform tests once there is a general agreement on this approach.

---

Preview | Diff

[christian-bromann]

any comments @shs96c @AutomatedTester ?

Added WPT tests in web-platform-tests/wpt#26490