refactor(tx-builder): extract common pointers code

[davidyuk]

This PR is supported by the Æternity Foundation

[codecov]

Codecov Report

Attention: Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.87%. Comparing base (1587d54) to head (29005ea).

Files	Patch %	Lines
src/tx/builder/field-types/pointers.ts	92.59%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1992      +/-   ##
===========================================
- Coverage    82.88%   82.87%   -0.02%     
===========================================
  Files          106      105       -1     
  Lines         3530     3522       -8     
  Branches       731      732       +1     
===========================================
- Hits          2926     2919       -7     
- Misses         303      304       +1     
+ Partials       301      299       -2     

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