Ability to insert secretgen-generated passwords into a helmTemplate step in a PackageInstall

**Describe the problem/challenge you have**
_Reporting on the behalf of @voor._
I want the ability to pass values to ytt and helm for Packages. This works for App CRs but not PackageInstall.

**Describe the solution you'd like**
I want to insert SecretGen generated passwords into a helmTemplate step in a PackageInstall. I see how I could do that with:
```
- helmTemplate:
...
valuesFrom:
- secretRef:
name: secret-name
```
Just basically put that into `spec.template.spec` for the Package, except I don't know what the secret-name would be ahead of time, and I need to support multiple installs of the package into the same namespace.

**Anything else you would like to add:**
Please take a look at the [initial conversation](https://vmware.slack.com/archives/CUK1X0Z8Q/p1642798354086500) or reach out to @voor directly. This may require work in secretgen-controller.

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