Missing securityContext attributes on container kube-rbac-proxy (open…

…search-project#848)

### Description
Allows the following setup

```yaml
kubeRbacProxy:
  securityContext:
    allowPrivilegeEscalation: false
    readOnlyRootFilesystem: true
    capabilities:
      drop:
      - all
```

### Issues Resolved
Fixes
[opensearch-project#745](opensearch-project#745)

### Check List
- [x] Commits are signed per the DCO using --signoff
- [ ] Unittest added for the new/changed functionality and all unit
tests are successful
- [ ] Customer-visible features documented
- [ ] No linter warnings (`make lint`)

If CRDs are changed:
- [ ] CRD YAMLs updated (`make manifests`) and also copied into the helm
chart
- [ ] Changes to CRDs documented

Please refer to the [PR
guidelines](https://github.com/opensearch-project/opensearch-k8s-operator/blob/main/docs/developing.md#submitting-a-pr)
before submitting this pull request.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and
signing off your commits, please check
[here](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).

Signed-off-by: Casper Thygesen <[email protected]>
Signed-off-by: rkthtrifork <[email protected]>

charts/opensearch-operator/values.yaml

@@ -74,7 +74,11 @@ serviceAccount:
 kubeRbacProxy:
   enable: true
   securityContext:
-    # allowPrivilegeEscalation: false
+    allowPrivilegeEscalation: false
+    readOnlyRootFilesystem: true
+    capabilities:
+      drop:
+        - ALL
   resources:
     limits:
       cpu: 50m