feat: add a Sequence Number setting

[overcat]

LedgerHQ/app-stellar#40

[codecov-commenter]

Codecov Report

Merging #13 (abd442a) into develop (1395998) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #13   +/-   ##
========================================
  Coverage    88.52%   88.52%           
========================================
  Files           12       12           
  Lines         2352     2352           
========================================
  Hits          2082     2082           
  Misses         270      270           

Flag	Coverage Δ
unittests	88.52% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/transaction/transaction_formatter.c	93.33% <ø> (ø)

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