native: add events to oracle contract

[roman-khimov]

Follow neo-project/neo#2036.

[codecov]

Codecov Report

Merging #1521 into master will increase coverage by 7.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1521      +/-   ##
==========================================
+ Coverage   70.59%   77.60%   +7.01%     
==========================================
  Files         229      229              
  Lines       17872    17896      +24     
==========================================
+ Hits        12617    13889    +1272     
+ Misses       4462     3043    -1419     
- Partials      793      964     +171     

Impacted Files	Coverage Δ
pkg/core/native/oracle.go	75.34% <100.00%> (+3.09%)	⬆️
pkg/core/blockchain.go	73.73% <0.00%> (+0.35%)	⬆️
pkg/core/interop_system.go	61.38% <0.00%> (+0.38%)	⬆️
pkg/compiler/codegen.go	90.37% <0.00%> (+0.40%)	⬆️
pkg/rpc/server/server.go	78.03% <0.00%> (+0.60%)	⬆️
pkg/rpc/client/wsclient.go	82.70% <0.00%> (+1.50%)	⬆️
pkg/rpc/client/nep5.go	48.67% <0.00%> (+1.76%)	⬆️
pkg/core/native/native_nep5.go	73.71% <0.00%> (+2.56%)	⬆️
pkg/wallet/wallet.go	78.87% <0.00%> (+2.81%)	⬆️
pkg/wallet/account.go	89.15% <0.00%> (+3.61%)	⬆️
... and 24 more

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