add getter to get last payment Id made in the system

[eduv09]

Fixes #27

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.37%. Comparing base (75137ae) to head (7ae21de).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #31      +/-   ##
===========================================
+ Coverage    83.69%   84.37%   +0.67%     
===========================================
  Files            6        6              
  Lines           92       96       +4     
  Branches        24       24              
===========================================
+ Hits            77       81       +4     
  Misses          15       15              

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