Allow defining extra arbitrary objects

Including a field for defining additional arbitrary objects is a common practice that adds versatility to Helm deployments. It is used for example in all Bitnami charts (under a permissive Apache 2.0 license).

This avoids having to include extra YAML files to kubectl apply before doing the Helm installation and allows all config to be cohesively managed in values. It also simplifies the Helm chart because it reduces the need to add new values and variables for everything.

For example I was going to make a MR that would add two different options to define host certs and keys - by specifying the name of an existing secret that contains them, or creating a secret directly. But with extraObjects those two ways are basically the same and only one new variable (for the secret name) would need to be created.