Implement getnativecontracts RPC

[fyrchik]

Close #1680 .

[codecov]

Codecov Report

Merging #1724 (4f1bea0) into master (29b1581) will increase coverage by 0.00%.
The diff coverage is 83.22%.

@@           Coverage Diff           @@
##           master    #1724   +/-   ##
=======================================
  Coverage   82.41%   82.41%           
=======================================
  Files         270      270           
  Lines       21883    21893   +10     
=======================================
+ Hits        18034    18043    +9     
- Misses       2742     2743    +1     
  Partials     1107     1107           

Impacted Files	Coverage Δ
pkg/core/state/contract.go	81.15% <ø> (ø)
pkg/core/native/oracle.go	77.08% <63.63%> (ø)
pkg/rpc/client/rpc.go	86.12% <71.42%> (-0.31%)	⬇️
pkg/core/native/nonfungible.go	82.17% <76.92%> (ø)
pkg/core/native/policy.go	85.93% <78.57%> (ø)
pkg/core/native/name_service.go	81.81% <79.16%> (ø)
pkg/core/native/native_nep17.go	78.26% <87.50%> (ø)
pkg/core/native/native_neo.go	72.46% <88.00%> (ø)
pkg/core/native/management.go	87.74% <88.88%> (-0.05%)	⬇️
pkg/core/blockchain.go	76.74% <100.00%> (+0.07%)	⬆️
... and 9 more

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[roman-khimov]

I feel a little uncomfortable without fixed-input client test, but it should be sufficient for now.