make proxy httpNumThreads configurable (#251)

Fixes https://github.com/apache/pulsar-helm-chart/issues/250

### Motivation

`httpNumThreads` is hardcoded to 8 in `charts/pulsar/templates/proxy-configmap.yaml`
When trying to override in `values.yaml` by using `proxy.configData.httpNumThreads` we get an error because the keys get duplicated.
This happens because `{{ toYaml .Values.proxy.configData | indent 2 }}` doesn't deduplicate the keys and there is no other way to set `httpNumThreads`

### Modifications

Removing the key from charts/pulsar/templates/proxy-configmap.yaml and adding it to the values yaml solves the problem.

### Verifying this change

- [x] Make sure that the change passes the CI checks.

charts/pulsar/templates/proxy-configmap.yaml

@@ -28,7 +28,6 @@ metadata:
     component: {{ .Values.proxy.component }}
 data:
   clusterName: {{ template "pulsar.cluster.name" . }}
-  httpNumThreads: "8"
   statusFilePath: "{{ template "pulsar.home" . }}/status"
   # prometheus needs to access /metrics endpoint
   webServicePort: "{{ .Values.proxy.ports.http }}"

charts/pulsar/values.yaml

@@ -889,6 +889,7 @@ proxy:
       -XX:-ResizePLAB
       -XX:+ExitOnOutOfMemoryError
       -XX:+PerfDisableSharedMem
+    httpNumThreads: "8"
   ## Add a custom command to the start up process of the proxy pods (e.g. update-ca-certificates, jvm commands, etc)
   additionalCommand:
   ## Proxy service