[ISSUE-133] add go module support

[zouzias]

Fixes #133

Simply adds the go.mod / go.sum files.

After merging the PR, a contributor should push a Git tag of the form vX.Y.Z to allow versioning.

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

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[zouzias]

@googlebot I signed it!

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[googlebot]

CLAs look good, thanks!

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