skaffold verify should support env vars with templating

The `skaffold.yaml` API for `verify` container only documents usage for name, image, command and args. This is insufficient for us to specify our tests which need env vars and need to be able to template values based on the environment that is running skaffold, similar to what we can do for our deployed helm charts.

We would like to do something like:

```yaml
verify:
container:
- name: foo
env:
- name: MY_STATIC_ENV
value: "Hello from the environment"
- name: MY_TEMPLATE_ENV
value: "Hello from {{.MY_EXTERNAL_VAR}}"
- name: MY_TEST_IMAGE
value: "{{.IMAGE_FULLY_QUALIFIED_my-test-image}}"
```

### Information

- Skaffold version: 2.13.0
- Operating system: Linux
- Installed via: skaffold.dev
- Contents of skaffold.yaml: see above