Multiple s3 provider support

[tchinmai7]

How to categorize this PR?

/area backup
/area disaster-recovery
/kind enhancement

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #914

Special notes for your reviewer:
Please read the discussion in the issue

Release note:

* Operators can now configure a secondary target for backup storage

[CLAassistant]

All committers have signed the CLA.

[gardener-robot]

@tchinmai7 Labels area/todo, kind/todo do not exist.

[gardener-prow]

Hi @tchinmai7. Thanks for your PR.

I'm waiting for a gardener member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[gardener-robot-ci-2]

Thank you @tchinmai7 for your contribution. Before I can start building your PR, a member of the organization must set the required label(s) {'reviewed/ok-to-test'}. Once started, you can check the build status in the PR checks section below.

[ishan16696]

can you please also add a document for this feature, so that it will be easy for us to follow ... may be here: https://github.com/gardener/etcd-backup-restore/tree/master/docs/usage with any relevant name.

[tchinmai7]

can you please also add a document for this feature, so that it will be easy for us to follow ... may be here: https://github.com/gardener/etcd-backup-restore/tree/master/docs/usage with any relevant name.

added in 9694aec

[tchinmai7]

@ishan16696 did you get a chance to review?

[ishan16696]

Hi @tchinmai7 ,
I didn't get a time to review it, I will be able to review it next week, sorry for the delay.

[renormalize]

/assign