core: add clone method to types

[corverroos]

Adds the Clone method to SignedData and other core types. This makes explicit that data should be cloned before sharing to ensure immutability.

category: refactor
ticket: #698

[codecov]

Codecov Report

Merging #706 (5b2e242) into main (0fc5059) will decrease coverage by 0.04%.
The diff coverage is 41.02%.

@@            Coverage Diff             @@
##             main     #706      +/-   ##
==========================================
- Coverage   54.47%   54.42%   -0.05%     
==========================================
  Files         103      103              
  Lines        9913     9971      +58     
==========================================
+ Hits         5400     5427      +27     
- Misses       3727     3760      +33     
+ Partials      786      784       -2     

Impacted Files	Coverage Δ
core/types.go	25.00% <0.00%> (-7.26%)	⬇️
core/sigagg/sigagg.go	52.38% <25.00%> (-4.03%)	⬇️
core/signeddata.go	41.42% <42.85%> (-0.07%)	⬇️
core/aggsigdb/memory.go	78.89% <50.00%> (-2.06%)	⬇️
core/validatorapi/validatorapi.go	50.87% <72.72%> (+0.36%)	⬆️
app/app.go	60.50% <0.00%> (+2.50%)	⬆️

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