Add support for custom endpoints in S3 snapstore

[sibucan]

What this PR does / why we need it:
Adds the capability to specify a different S3 endpoint within the S3 snapstore logic. This allows usage of etcd-backup-restore with non-AWS S3-compliant providers, such as min.io or Linode Object Storage. It also adds a way of setting S3ForcePathStyle, which is needed for etcd-backup-restore to be able to save the snapshot in these S3-compatible providers.

Note that by default when using the AWS SDK client, providing an empty string in the Endpoint field of the aws.Config{} struct is functionally similar to what was being done before (which was keeping it nil), and will generate the default endpoint, see: https://pkg.go.dev/github.com/aws/aws-sdk-go@v1.22.0/aws?utm_source=gopls#Config

Last but not least, I understand there's at least one PR that may modify the logic in this one. I am willing to rebase this PR if necessary if that one gets merged first.

Which issue(s) this PR fixes:
Fixes #416

Special notes for your reviewer:

Release note:

* Added support for non-AWS S3-compatible providers by specifying a custom endpoint.

[gardener-robot]

@sibucan Thank you for your contribution.

[gardener-robot-ci-2]

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[timuthy]

Thanks for the contribution. One comment from my side, the rest looks good to me.

[timuthy]

@ishan16696 are you fine with rebasing #428, so that we can merge this one first?

[ishan16696]

I think we should go with #435 first then #436 followed by this PR as this PR also required to update the doc as well as Plamen and I don’t have to retest #435 PR with 5 different storage providers.

[timuthy]

@sibucan kindly asking you to rebase this PR 🙂

[ishan16696]

@sibucan can you also update the document for storage providers accordingly.
Thanks !!

[sibucan]

@timuthy I've rebased and updated the document as requested -- one thing to note is that I have not changed my code so that the AWS endpoint can be configured via a file, since this functionality is freshly-merged and I wasn't sure if my changes should also follow suit (meaning they should also be configurable via a file). I guess it can also be added in a subsequent PR if deemed necessary.

[timuthy]

/lgtm