core: add ability to create hard-forks

[AnnaShaleva]

Close #2464.

[codecov]

Codecov Report

Merging #2469 (4d4f616) into master (48ee33f) will decrease coverage by 0.03%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master    #2469      +/-   ##
==========================================
- Coverage   85.10%   85.07%   -0.04%     
==========================================
  Files         291      293       +2     
  Lines       36804    36868      +64     
==========================================
+ Hits        31321    31364      +43     
- Misses       4171     4186      +15     
- Partials     1312     1318       +6     

Impacted Files	Coverage Δ
pkg/core/interops.go	100.00% <ø> (ø)
pkg/rpc/response/result/version.go	87.14% <ø> (ø)
pkg/config/hardfork_string.go	50.00% <50.00%> (ø)
pkg/core/interop_system.go	92.93% <70.00%> (-2.80%)	⬇️
pkg/config/hardfork.go	100.00% <100.00%> (ø)
pkg/config/protocol_config.go	85.54% <100.00%> (+0.73%)	⬆️
pkg/core/blockchain.go	82.31% <100.00%> (+0.04%)	⬆️
pkg/core/interop/context.go	91.89% <100.00%> (+0.36%)	⬆️
pkg/network/payload/mptdata.go	81.25% <0.00%> (-18.75%)	⬇️
pkg/network/compress.go	64.00% <0.00%> (-12.00%)	⬇️
... and 6 more

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[AnnaShaleva]

Tests are fixed.