Add support for GCS.

[lesaux]

Description of the Change

Adding support for GCS buckets.

Implementation trying to follow how it was done for s3.

Checklist

• allow Maintainers to edit PR (rebase, run coverage, help with tests, ...)
• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• [] author name in AUTHORS

[neolynx]

Hi !

great work :-)

I tried to fix the pipeline issue on master, but was not able to rebase the branch on your repo. could you give maintainer access to your PR ?

[lesaux]

Hi !

great work :-)

I tried to fix the pipeline issue on master, but was not able to rebase the branch on your repo. could you give maintainer access to your PR ?

Thank you!

I added you as a maintainer on my fork.