[Feature] Add SLA metrics to Intercom reports

### Is there an existing feature request for this?

- [X] I have searched the existing issues

### Describe the Feature

SLA metrics are becoming a more crucial reporting mechanism for Intercom customers, particularly with how customer success teams evaluate the effectiveness of their teammates.

This feature would add the following data:

- Add `median_next_response_time` metric to `intercom_conversations` model, would need to be calculated upstream in `intercom_conversation_parts` model. (This could be the heaviest lift, so open to discussing).
- Configurable variables for `target_first_response_time_minutes` and `target_next_response_time_minutes` for the customer and set, to allow us to define whether SLAs get hit or not in our models.
- Add `is_sla_hit` or `is_sla_miss` boolean fields based on `target_first_response_time_minutes` and `target_next_response_time_minutes` configurable variables in `conversation_metrics.
- Add `sla_hits` and `sla_misses` to eventually calculate SLA hits and misses by admin and company, to `admin_metrics` and `company_metrics`.

### Describe alternatives you've considered

The SLA metrics we get directly from Intercom do have 'hit', 'miss', 'cancelled' values according to documentation. So we could technically get aggregate SLA hit and miss rate over a particular time period if we want to scope down.

However, we do not know what the actual SLA targets are, customer-by-customer, as they are configurable goals that could very much differ. It'd be better if we set SLA goal metrics on our end based on targets.

If `median_next_response_time` is too complicated to calculate, it might be worth discussing whether we parse down this to request to only the `first_response` SLA targets.

### Are you interested in contributing this feature?

- [X] Yes.
- [ ] Yes, but I will need assistance and will schedule time during your [office hours](https://calendly.com/fivetran-solutions-team/fivetran-solutions-team-office-hours) for guidance.
- [ ] No.

### Anything else?

[Read more on Intercom SLA performance here.](https://www.intercom.com/help/en/articles/2672572-measure-your-sla-performance)