docstore/all: add Offset method implementation

[bartventer]

This pull request adds an Offset method to the docstore package for all providers. This method allows users to skip a specified number of documents in the results of a query, which is useful for implementing pagination in applications that use docstore. This complements the existing Limit method.

I've tested this implementation across all providers and generated new replay files for awsdynamodb and gcpfirestore.

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.15%. Comparing base (5ec203b) to head (ce7cb1b).
Report is 2 commits behind head on master.

Files	Patch %	Lines
docstore/query.go	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3385      +/-   ##
==========================================
- Coverage   73.18%   73.15%   -0.04%     
==========================================
  Files         113      113              
  Lines       14792    14846      +54     
==========================================
+ Hits        10826    10860      +34     
- Misses       3202     3213      +11     
- Partials      764      773       +9     

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[vangent]

Thanks for the contribution!