Export default native contract constructors

[AnnaShaleva]

Problem

nspcc-dev/neofs-node#3759 (comment). Metadata chain needs some of default native implementations.

Solution

Export Policy and Notary constructors. I don't want to export Neo because this implementation contains too many N3-specific mechanics which is not needed in metadata chain at all and should be mocked on the NeoFS side by custom (and simple) native Neo. Ref. nspcc-dev/neofs-node#3759 (comment).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.38%. Comparing base (fc4a77d) to head (6e46431).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4145      +/-   ##
==========================================
- Coverage   83.39%   83.38%   -0.02%     
==========================================
  Files         356      356              
  Lines       43146    43146              
==========================================
- Hits        35982    35976       -6     
- Misses       5407     5409       +2     
- Partials     1757     1761       +4     

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