Retain platform-specific timestamp field that are being added in bug 1961012

[mstange]

This PR makes us carry the fields added in bug 1961012 through into the processed format, and adds them to the type definitions.

The corresponding change to samply's simpleperf importer is in mstange/samply#560 .

I'm making use of the startTimeAsClockMonotonicNanosecondsSinceBoot field in the merging script which I've put into #5448.

Pasting the justification from the bug here:

I would like to combine Gecko profiler marker data with Android simpleperf sample data. To make a combined profile, I need to adjust timestamps so that they're in the same space.

simpleperf uses CLOCK_MONOTONIC_RAW timestamps.
The Gecko profiler uses the TimeStamp class which, on Android, stores CLOCK_MONOTONIC_RAW values internally.
But in the profile JSON, we lose this information: profile.meta.startTime is in float seconds since the Unix epoch, and the rest of the time values is relative to the startTime.

I think it would be useful to have the CLOCK_MONOTONIC_RAW startTime in the profile JSON.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.03%. Comparing base (8b83c7a) to head (f23392b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5446   +/-   ##
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  Coverage   86.03%   86.03%           
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  Files         312      312           
  Lines       30365    30365           
  Branches     8299     8299           
=======================================
  Hits        26126    26126           
  Misses       3645     3645           
  Partials      594      594           

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[julienw]

thank you