Provide offline installation bundle

[stephan2012]

While the Helm S3 plugin is very helpful it seems to be available for online installation only (i.e., requires an Internet connection). We are going to use Helm on Internet connected systems as well as offline (air-gap) systems with private, non AWS-provided S3 buckets. For this reason it would be helpful to have an offline installation package for every release. This would also help in pinning a tested version.

[hypnoglow]

I believe the offline installation is "doable" if you follow the Development guide: clone the repo, install dependencies, build the binary and execute the installation command from the guide.

But I agree that we could provide a more convenient way for this case, bundling the binary and all required plugin files (which I think already present in "releases" section of this repo) and documenting how to install it without a public internet connection.

[horacimacias]

I'm not sure how releases are being produced and uploaded to github but here's what I did manually and I think can be used.

1. Download the release tgz
2. Unarchive on some directory
3. Edit plugin.yaml and remove the "hooks" section. The local directory now has the edited plugin.yaml and the bin/helms3 that came with the tgz
4. Run helm plugin install . This seems to do it for me

An option is to produce "-offline" release tgz files with the modified plugin.yaml file, so people can download that and install the plugin offline. They will need to unarchive the tgz but I think that is not an issue for somebody configuring an offline install anyway.