core: introduce whitelisted contracts

[AnnaShaleva]

Close #4039.

TODO:

• Compatibility test for whitelisted item key.
• Unit-tests for interop/RPC client APIs.
• Docs update.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.42%. Comparing base (0fd6ffe) to head (29d506a).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/core/native/policy.go	80.40%	25 Missing and 4 partials ⚠️
pkg/rpcclient/policy/policy.go	83.33%	7 Missing and 7 partials ⚠️
pkg/core/interop/contract/call.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4052      +/-   ##
==========================================
+ Coverage   72.60%   74.42%   +1.81%     
==========================================
  Files         364      364              
  Lines       56739    47833    -8906     
==========================================
- Hits        41195    35598    -5597     
+ Misses      13816    10495    -3321     
- Partials     1728     1740      +12     

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