refactor: move ValidationState to its own file

[glevco]

Acceptance Criteria

• Move ValidationState class to its own file

Reason: cleaning up code to prepare for Transaction-related refactors

[codecov]

Codecov Report

Merging #635 (29734e5) into master (b2bb354) will decrease coverage by 0.03%.
The diff coverage is 94.28%.

@@            Coverage Diff             @@
##           master     #635      +/-   ##
==========================================
- Coverage   83.94%   83.91%   -0.03%     
==========================================
  Files         240      241       +1     
  Lines       20126    20128       +2     
  Branches     2743     2743              
==========================================
- Hits        16894    16891       -3     
- Misses       2636     2638       +2     
- Partials      596      599       +3     

Impacted Files	Coverage Δ
hathor/transaction/validation_state.py	93.54% <93.54%> (ø)
hathor/transaction/base_transaction.py	91.24% <100.00%> (+0.01%)	⬆️
hathor/transaction/storage/transaction_storage.py	91.04% <100.00%> (-0.14%)	⬇️
hathor/transaction/transaction_metadata.py	90.44% <100.00%> (-0.47%)	⬇️

... and 2 files with indirect coverage changes