Add Cert-Manager backend

[nightkr]

Builds on #473 by automatically creating the applicable Cert-Manager Certificate objects, and mounting in the applicable certificates.

[lfrancke]

Can you please add a link to the documentation as well as a short snippet for the release notes?

And would it be useful to show this off to the team?

[nightkr]

Docs: https://docs.stackable.tech/home/nightly/secret-operator/cert-manager

Relnote: "The Stackable Data Platform now supports provisioning TLS certificates using cert-manager."

And would it be useful to show this off to the team?

Maybe? Probably worth mentioning that it exists, but as an operator dev it should be transparent.