Add the ability to add Environment variables as a configmap

Statefulsets have the option to add Environment variables as configmaps like so...

```
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: my-statefulset
spec:
replicas: 1
template:
spec:
containers:
- name: my-app-container
image: your-image:tag
envFrom:
- configMapRef:
name: my-app-config
```

Currently the SolrCloud config [only supports adding additional EnvVariables in line.](https://github.com/apache/solr-operator/blob/8e986e60a82fcd577fe55e882e23a5491f4d3014/api/v1beta1/common_types.go#L78)

The existing API field is in api/v1beta1/common_types.go; start there and trace how SolrCloud environment settings are rendered into the StatefulSet. Verify that the CRD accepts a ConfigMap reference like the example and that the generated StatefulSet contains the corresponding envFrom entry.

Written by the indexing model from the issue text.