[plugin-barman-cloud] [blocker] certificates in chart are broken

Hey folks.

I've tried to use the plugin-barman-cloud chart but I'm struggling to get the certificates correctly. The way it is implemented now it is:

1. Forcing usage of cert-manager: Yes, that is the common way on kubernetes to get certificates but there some people (already saw some complains on other issues) which don't use it. So have a way to provide a secret references is needed.
2. For the cert-manager users: The chart is creating its own cert-manager self-signed issuer regardless if the user specify a name of an existent issuer.

To fix this I suggest:

1. Have a value where the user can pass the TLS secret name for both the server and client: If this is specified, nothing else regarding certificates will be used since the user explicitly want to use those certificates
2. Have a issuerName value: if passed, will request both server and client certificates from cert manager

If none are provided the chart should fail to deploy as TLS is required.

Does that make sense? If so, I can put a PR quickly to fix it. The way it is right now the chart is completely useless since the certificates will be either generated on an untrusted (self-signed) CA or not created at all.

Thanks!