Create support for Marker Schema coming from Gecko

This is dependent on #2731 being done first to land. It can be developed on the side.

In the GeckoProfile, there are multiple `process.meta.markerSchema` entries, one for each process. The `MarkerSchema[]` will ONLY include the markers used in that process. These individual entries will need to be collected into a global marker schema list, that is de-duplicated on the key. This will be stored in the unified `ProfileMeta` object in the processed format.

In addition, the `getMarkerSchema` selector will need to be updated to first look for the marker schema, and then fallback to one on the front-end if it exists.

This work should be coordinated with a version bump on the Gecko side, so that we can start landing the MarkerSchema now, and worry about it being 100% correct only later on. This work should be merged when we are ready to launch.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-302)