Allow VMAlert CRD to store s3 credentials in kubernetes secret

[tiny-pangolin]

I would like the ability to store the access key and key_id as a kubernetes secret similar to how it is done in backup manager. It would nice if CustomS3Enpoint was also added so specifying a custom is consistent across CRDs

[ngxiaoen]

Hi, I'd like to help with this issue if it's still relevant?

A couple of things I'd like to confirm on scope:

• Would a docs update be enough? to show the existing extraEnvs + secretKeyRef (for credentials) and extraArgs (for -s3.customEndpoint)
• Or a dedicated credentialsSecret / customS3Endpoint field added to VMAlertSpec, for consistency with vmBackup on the VMCluster/VMSingle?

Happy to help either way as I use victoriametrics