[ResponseOps][Cases] Track metrics regarding status changes in the cases SO

[cnasikas]

As part of the #216259 effort we need to track the following metrics in the Cases SO:

1. time_to_acknowledge: The time in seconds taken to mark the case as in progress. It is calculated as case.in_progress_at - case.created_at.
2. time_to_investigate: The time in seconds taken to mark the case from in progress to close. It is calculated as case.closed_at - case.in_progress_at.
3. time_to_resolve: The time in seconds taken to close a case. It is calculated as case.closed_at - case.created_at.

DoD

• Create a new model version and add the metric fields.
• Let the APIs return the metrics.
• Update the time_to_acknowledge field when someones mark a case as in progress.
• Update the time_to_resolve field when someones closes a case.
• Update the time_to_investigate field when someones closes a case.

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)