Expose activated_at for session details

[mriedem]

At some point (hopefully in the near future) the cloud API should return an activated_at timestamp field and the quantum API should return an activatedAtactivated_at field. Those will indicate when the session last went into the active state and can be used with the active_ttl to calculate how much time is remaining in the active window for the session (assuming the session has more jobs to run and doesn't go inactive after the interactive_ttl expires).

Originally posted by @mriedem in #1119 (comment)

[mriedem]

The cloud API now exposes activated_at in staging:

https://us-east.quantum-computing.test.cloud.ibm.com/openapi/#/Sessions/get_session_information

Should be in production next week.

The IQP API already exposes activated_at:

https://api.quantum-computing.ibm.com/runtime/api#/sessions/GetSessionDetailsController_getSessionDetails