Make various GC markers schema-based

Markers with types `GCSlice`, `GCMinor` and `GCMajor` currently have custom code for their tooltips. Their fields `timings` / `nursery` / `nurseryTimings` are not declared in a schema.

We should add marker field formats for these JSON objects and declare the fields in the schema, for #5302.

So then the custom tooltip code would still be there, but it would just be called for a field whose format is e.g. `gc-minor-nursery-timings`, rather than for a marker whose `data.type` is `GCMinor`.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-855)