Explore more predictable solutions for `previous_*` field values

In [pendo__page_event](https://github.com/fivetran/dbt_pendo/blob/main/models/pendo__page_event.sql#L30-L33) and [pendo__feature_event](https://github.com/fivetran/dbt_pendo/blob/main/models/pendo__feature_event.sql#L29-L32), we have the following kind of logic for grabbing previous page/feature event values:

```sql
lag() over(partition by visitor_id order by occurred_at asc, _fivetran_synced asc) as previous_
```

However, events will often have identical `occurred_at` and `_fivetran_synced` timestamps, so SQL arbitrarily chooses the previous event.

Let's explore other solutions to produce consistent outputs -- perhaps there have been new fields added since we first made these models that we can leverage.

Start with models/pendo__page_event.sql lines 30-33 and models/pendo__feature_event.sql lines 29-32, then inspect the available Pendo event fields for a stable ordering value. Compare outputs for events sharing both timestamps; done means the previous page and feature values are produced consistently for tied events.

Written by the indexing model from the issue text.