Better support for COSI

Currently the ObjectStore expects certain details to be statically configured like the endpoint url and the bucket name as part of the destination path. This is perfectly fine when generating buckets manually, but it requires additional tooling when working with dynamically provisioned buckets using COSI or other operators like e.g. [minio-operator-ext](https://github.com/benfiola/minio-operator-ext).

In a perfect world ObjectStore would simply accept a reference to a COSI BucketAccess object, but I understand that COSI is still in alpha state and object storage provider support is pretty limited and this wouldn't support other operators.

A simpler way to at least improve the situation would be to accept more values as secretKeyRefs such as the accessKeyId and secretAccessKey objects of the s3Credentials objects. COSI puts all information about necessary to access a bucket into a secret.