[DGS-2634] add BatchSendings and BatchSendingsItems endpoints

[PavelVais]

• Implement BatchSendingsEndpoint for managing batch sendings.
• Implement BatchSendingsItemsEndpoint for handling items within a batch.
• Create BatchSending resource to represent batch sending data.
• Add tests for BatchSendings and BatchSendingsItems endpoints.
• Update DigiSign class to include batchSendings method.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.52%. Comparing base (1ac1004) to head (b7c3f7f).
Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x     #256      +/-   ##
============================================
+ Coverage     97.49%   97.52%   +0.02%     
- Complexity      462      467       +5     
============================================
  Files            72       74       +2     
  Lines          1079     1089      +10     
============================================
+ Hits           1052     1062      +10     
  Misses           27       27              

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[spajxo]

špatný typ