feat(feature-activation): implement signal_bits field in txs

[glevco]

Acceptance Criteria

• Change TxVersion enum so it fails instantiation for values larger than one byte, instead of ignoring it.
• Create new signal_bits attribute in BaseTransaction, derived from the first byte of the version field.

ATTENTION: this PR touches important parts of the code

[codecov]

Codecov Report

Merging #623 (dfd7755) into master (b01ab74) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
- Coverage   83.94%   83.92%   -0.03%     
==========================================
  Files         241      241              
  Lines       20128    20134       +6     
  Branches     2743     2742       -1     
==========================================
+ Hits        16896    16897       +1     
- Misses       2636     2638       +2     
- Partials      596      599       +3     

Impacted Files	Coverage Δ
hathor/transaction/base_transaction.py	91.28% <100.00%> (+0.03%)	⬆️
hathor/transaction/block.py	90.71% <100.00%> (+0.05%)	⬆️
hathor/transaction/merge_mined_block.py	76.74% <100.00%> (ø)
hathor/transaction/token_creation_tx.py	99.30% <100.00%> (+<0.01%)	⬆️
hathor/transaction/transaction.py	92.66% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes