fix: decompressive transcoding improvements

[coryan]

Store the data in compressed form (if applicable), I believe this is a
better approach as it:

• Makes the size and checksum attributes match the behavior in the
  service.
• It enables support for Accept-Encoding, we can return compressed
  data.
• It makes the gRPC case match the service behavior, i.e., always return
  compressed data.

Fixes #321

[codecov]

Codecov Report

Merging #322 (d3844e1) into main (465f85e) will increase coverage by 0.07%.
The diff coverage is 99.27%.

@@            Coverage Diff             @@
##             main     #322      +/-   ##
==========================================
+ Coverage   92.23%   92.30%   +0.07%     
==========================================
  Files          48       47       -1     
  Lines        7627     7724      +97     
==========================================
+ Hits         7035     7130      +95     
- Misses        592      594       +2     

Flag	Coverage Δ
unittests	92.30% <99.27%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
testbench/__init__.py	100.00% <ø> (ø)
tests/test_testbench_object_gzip.py	99.00% <99.00%> (ø)
gcs/object.py	98.11% <100.00%> (-0.57%)	⬇️
testbench/common.py	98.67% <100.00%> (+<0.01%)	⬆️
testbench/rest_server.py	98.67% <100.00%> (+<0.01%)	⬆️
tests/format_multipart_upload.py	100.00% <100.00%> (ø)

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[tritone]

Very nice improvement, thank you!