core: add message root to signed data interface

[corverroos]

Add explicit unsigned data message root function to signed data as a better way to ensure consistent unsigned data in parsigdb. Relying on json serialisation to check for consistent signed data is brittle.

category: refactor
ticket: #1347

[codecov]

Codecov Report

Base: 53.67% // Head: 53.74% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (e19186f) compared to base (6d1ba91).
Patch coverage: 42.10% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1349      +/-   ##
==========================================
+ Coverage   53.67%   53.74%   +0.06%     
==========================================
  Files         143      142       -1     
  Lines       17049    17137      +88     
==========================================
+ Hits         9151     9210      +59     
- Misses       6624     6652      +28     
- Partials     1274     1275       +1     

Impacted Files	Coverage Δ
core/types.go	28.91% <ø> (ø)
core/signeddata.go	41.37% <42.10%> (+0.05%)	⬆️
core/priority/prioritiser.go	61.65% <0.00%> (-3.11%)	⬇️
core/qbft/qbft.go	81.54% <0.00%> (-0.43%)	⬇️
core/tracker/component_string.go
core/scheduler/scheduler.go	73.47% <0.00%> (+0.53%)	⬆️
core/dutydb/memory.go	67.22% <0.00%> (+0.84%)	⬆️
core/tracker/tracker.go	73.57% <0.00%> (+1.52%)	⬆️
app/vmock.go	75.40% <0.00%> (+1.60%)	⬆️

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[dB2510]

I was also thinking about the same in line with #920. This will simplify verification may also help in refactoring verification logic into single function.