feat(helm): allow an user defined existing secret

[cebidhem]

Hey there,

We are using Trivy for a long time in our CI, but we recently decided to give a try to the Trivy Operator, and therefore we want to deploy Trivy as a server first.
Looking at the Helm chart, all secrets must be defined as values in order to get consumed by the statefulset. It would be nice to have a way to define a secret created outside of the chart (we're using external-secrets but it could be something else).

I will submit a PR on this since I haven't seen any, and this should be pretty quick to do.

Let me know if it was on purpose, I'll close.