Make MinIO storage optional

[elichad]

We originally added MinIO storage for two reasons:

1. the CRS4 validator required loading the metadata from a file, MinIO allowed us to store crates to meet this need
2. opens options for validating the contents of the crate

However:

1. the CRS4 validator can now accept JSON directly as input, so the crate does not need to be saved to file any more
2. we aren't doing contents validation at the moment and it doesn't appear that there is a need for this in the short-term future
3. having an object store in production means additional security considerations, especially as this would (currently) live separately to the rest of the 5S-TES or K8TRE stacks

Therefore, we should make the MinIO serve an optional component of this stack. This would mean that by default the service would not store anything, but this could still be enabled through a setting.