Allow for specifying a custom CA certificate in a Bucket

[makkes]

As a user that has a custom S3 implementation such as MinIO running in my cluster, I want to be able to use an encrypted connection between s-c and the S3 server. For that to work, I need to be able to define a custom CA certificate in the caFile field of the Secret referred to in the .spec.secretRef. Using the caFile field is in line with other resources such as GitRepositories and {HelmRepositories](https://fluxcd.io/flux/components/source/helmrepositories/#tls-authentication).