Enhancement/bk envelope

[theflyingcodr]

This replaces the JSONEnvelope and validation code within minercraft with the github.com/libsv/go-bk implementation.

Unit tests have also been added to check for correct mapping and validation of the wrapper struct.

[codecov]

Codecov Report

Merging #30 (3373e15) into master (8c5383d) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head 3373e15 differs from pull request most recent head 8c78345. Consider uploading reports for the commit 8c78345 to get more accurate results

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   93.61%   93.53%   -0.08%     
==========================================
  Files           8        8              
  Lines         329      325       -4     
==========================================
- Hits          308      304       -4     
  Misses         15       15              
  Partials        6        6              

Impacted Files	Coverage Δ
definitions.go	100.00% <100.00%> (ø)

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[mrz1836]

LGTM