Support snapshots in S3

[siegfriedweber]

Part #1

Tasks

• test: Test snapshots in S3 #49
• Decision: Allow adding secrets to the OpenSearch keystore #50
• feat: Allow adding entries to the OpenSearch keystore #76

[lfrancke]

I don't know enoug about OpenSearch to judge: Should this be mentioned in the release notes?

[labrenbe]

I think it's enough that we have a release notes entry for #76 as the rest of the issue is only a decision & integration test.