Add backend for Google Cloud Storage (GCS)

[fd0]

Main work by @diptadas, thank you very much! Successor of #1052.

TODO:

• Move files to new repo structure
• Vendor dependencies with dep
• Configure backend tests
• Test the backend thoroughly
• Add documentation to the manual (see Add Google Cloud Storage as backend #1052 (comment))

Closes #211

[xor-gate]

Is the GCS the same as Gdrive or is it different?

[fd0]

It's entirely different. GCS is similar to S3.

[xor-gate]

I see, the tool rclone (golang) tool does support gdrive maybe we can find a way to create also a gdrive backend. But thanks for your answer, I will continue in a different thread (#527)

[codecov-io]

Codecov Report

Merging #1134 into master will increase coverage by 0.11%.
The diff coverage is 57.51%.

@@            Coverage Diff             @@
##           master    #1134      +/-   ##
==========================================
+ Coverage   52.71%   52.83%   +0.11%     
==========================================
  Files         122      124       +2     
  Lines       11454    11760     +306     
==========================================
+ Hits         6038     6213     +175     
- Misses       4708     4823     +115     
- Partials      708      724      +16

Impacted Files	Coverage Δ
internal/backend/location/location.go	60% <ø> (ø)	⬆️
cmd/restic/global.go	21.25% <0%> (-1.65%)	⬇️
internal/backend/gs/gs.go	60.78% <60.78%> (ø)
internal/backend/gs/config.go	77.77% <77.77%> (ø)
internal/archiver/archiver.go	64.73% <0%> (-0.17%)	⬇️

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[cafuego]

I flipped my repo URLs to the new gs backend recently and backup speed increased by a factor of 4! Thanks for doing this work @diptadas and merging it @fd0! 🧡