Explore dynamic mapping for app logs datastream

The `app_logs` datastream is used for mapping OTEL logs and (after PR #9068) intake v2 logs. The datastream is currently configured with `dynamic: false` which disables automatic detection and mapping of new fields.

We want to explore setting `dynamic: runtime` on the field to enable fields to be searchable by default. One of the motivation for this is to enable greater flexibility for the logs intake support via APM-Server as ECS logging library may add arbitrary key/value pairs to the document root.

### Related issues
#8757 ([relevant comment](https://github.com/elastic/apm-server/issues/8757#issuecomment-1239170331))