Feature Request: Relax volume constraint to support more volume types

## In what area(s)?

/area API
> /area autoscale
> /area build
> /area monitoring
> /area networking
> /area test-and-release

## Describe the feature

We currently only allow for configmap and secret volumes to be mounted into the user container. This constraint is put in place as volumes are a source of state and can severely limit scaling. This feature request is to relax this constraint to allow a larger set of volumes to be mounted that work well with serverless functions.

I do not have a particular set of volume types in mind, but #4130 may be a good example.