Add session tracking

[igneel64]

Description

We can add AMP session tracking attributes to our collection.

Proposal

Add a new schema describing the AMP session and the available identifiers we can track.

The description and naming on the attributes available is as such from the [amp repository (https://github.com/ampproject/amphtml/issues/33990):

• Session_id: a low entropy value persisted for 30 minutes per domain per vendor
• Session Creation Timestamp: a timestamp is seconds since epoch when the current session started
• Session Count: starts at 1 and is incremented each time a new session starts
• Engaged: 0 if there has not yet been user engagement and 1 once user engagement is achieved (signaling that this is not a bounced session)
• last-event-timestamp: a timestamp in seconds since epoch when the most recent event was generated

From the above, the naming we chose to go about is:

• ampSessionId → Session_id
• ampSessionIndex → Session Count
• sessionEngaged → Engaged
• lastSessionEventTimestamp → last-event-timestamp
• sessionCreationTimestamp → Session Creation Timestamp

These would be implemented in a new context which would be carried over with every event, as we already do with ones like amp_id.