core: restrict allowed Oracle callbacks

[AnnaShaleva]

Changes ported from neo-project/neo#2159.

We haven't the same problem with _deploy invocations, but we have a bug with Oracle callbacks.

[codecov]

Codecov Report

Merging #1625 (31b0690) into master (f2365e2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1625      +/-   ##
==========================================
+ Coverage   82.56%   82.58%   +0.01%     
==========================================
  Files         241      241              
  Lines       19100    19102       +2     
==========================================
+ Hits        15770    15775       +5     
+ Misses       2341     2338       -3     
  Partials      989      989              

Impacted Files	Coverage Δ
pkg/core/native/oracle.go	81.64% <100.00%> (+0.17%)	⬆️
pkg/network/server.go	73.92% <0.00%> (+0.17%)	⬆️
pkg/rpc/client/wsclient.go	82.70% <0.00%> (+1.50%)	⬆️

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