Initial batch support

[oittaa]

Fixes #10

https://googleapis.dev/python/storage/latest/batch.html

https://cloud.google.com/storage/docs/json_api/v1/how-tos/batch

[codecov-io]

Codecov Report

Merging #12 (47a9761) into main (33dc70d) will increase coverage by 0.61%.
The diff coverage is 96.38%.

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   91.39%   92.00%   +0.61%     
==========================================
  Files           7        7              
  Lines         639      713      +74     
==========================================
+ Hits          584      656      +72     
- Misses         55       57       +2     

Flag	Coverage Δ
unittests	92.00% <96.38%> (+0.61%)	⬆️

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

Impacted Files	Coverage Δ
gcp_storage_emulator/handlers/objects.py	92.45% <94.64%> (+0.98%)	⬆️
gcp_storage_emulator/server.py	92.70% <100.00%> (+0.91%)	⬆️

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