Add new marker schema format: "pid" (process id)

[squelart]

In https://bugzilla.mozilla.org/show_bug.cgi?id=1757389 , we want to add a new mozilla::MarkerSchema::Format item corresponding to a "pid" (process id). Eventually (in this bug or a follow-up), the front-end could display these with the process name&type found elsewhere in the profile.

If this makes sense to do at the same time, we could also add a "tid" (thread id) format, instead of relying on the marker data property name to be exactly "threadId".

This issue here should be implemented before https://bugzilla.mozilla.org/show_bug.cgi?id=1757389 can land, please synchronize.