Allow pods to use other roles

**What would you like to be added**:

Allow pods to use other roles

**Why is this needed**:

Currently, to use other roles in pods I need to mount the token volume manually.

The code [1], does not add the env variables if they are already present, which is good, but it does not mount the volume [2]

* [[1]](https://github.com/aws/amazon-eks-pod-identity-webhook/blob/master/pkg/handler/handler.go#L103-L106)
* [[2]](https://github.com/aws/amazon-eks-pod-identity-webhook/blob/master/pkg/handler/handler.go#L128)

It would be nice if the volume is mounted even when the env vars are present.

This Feature Request is based on the discussion from #20

Read pkg/handler/handler.go at the linked references around lines 103-106 and 128, then review the discussion in issue #20 for the intended interaction between existing environment variables and volume mounting. Done means pods using other roles receive the token volume even when those environment variables are already present, without changing the existing environment-variable behavior.

Written by the indexing model from the issue text.