Support extensibility in the set of Layers 1

[dr-orlovsky]

This allows to add more Layer1/sidechains downstream, if required, without an API break here.

Continuation of work from #199

[codecov]

Codecov Report

Attention: Patch coverage is 7.56303% with 110 lines in your changes are missing coverage. Please review.

Project coverage is 15.1%. Comparing base (f50bf78) to head (aa1ae5f).

Files	Patch %	Lines
src/contract/xchain.rs	8.5%	97 Missing ⚠️
src/contract/seal.rs	0.0%	12 Missing ⚠️
src/contract/anchor.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #223     +/-   ##
========================================
- Coverage    15.3%   15.1%   -0.2%     
========================================
  Files          33      33             
  Lines        4218    4308     +90     
========================================
+ Hits          646     650      +4     
- Misses       3572    3658     +86     

Flag	Coverage Δ
rust	15.1% <7.6%> (-0.2%)	⬇️

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