Add a label to the S3User's Secret to allow label matching and speed up fetching the secret

As of current version, the user controller lists every secret in a namespace to find the one secret matching a S3User, which is fairly inefficient :

https://github.com/InseeFrLab/s3-operator/blob/57a7f12a60736fc695a5d8ba5204ba91524feea4/controllers/user_controller.go#L408-L438

This could benefit from a dedicated label add to the secret when it's created. This is not difficult in itself, but requires some thought regarding pre-existing secrets (as in : Should the operator reconcile secrets to add labels ? Should this be managed with a small one-shot script to add the label to every S3User secret ?)

EDIT : [usage example visible in Operator SDK doc](https://sdk.operatorframework.io/docs/building-operators/golang/references/client/#list)