[wishlist] support for setting securityContext for pods on deployment

I wish to deploy an application that includes an ipsec client, using Strongswan, in a sidecar charm.

In order to have that run, I need the container to be a privileged one - this can be achieved using a podspec, e.g.

```
apiVersion: apps/v1
kind: Deployment
metadata:
  name: vpn
  labels:
    app: vpn
spec:
  selector:
   matchLabels:
     app: vpn
  replicas: 1
  template:
    metadata:
     labels:
       app: vpn
    spec:
      containers:
      - name: ipsec
        image: xavpaice/ipsec-connector:0.1-edge
        imagePullPolicy: Always
        securityContext:
          privileged: true
        ports:
          - containerPort: 500
            hostPort: 500
            name: vpn-isakmp
            protocol: UDP
          - containerPort: 4500
            hostPort: 4500
            name: vpn-ike
            protocol: UDP
```

It would be good to have this definable in the metadata.yaml containers setting, rather than have to work around the limitation e.g. https://github.com/danackerson/wireguard-sidecar/blob/81bedf5d4c9299e96b9321fe342cb55bc0614a3c/src/charm.py#L103