adding service account for mqtt-client (#105)

## Purpose
<!-- Describe the intention of the changes being proposed. What problem
does it solve or functionality does it add? -->
* adding service account for mqtt-client

## Does this introduce a breaking change?
<!-- Mark one with an "x". -->
```
[ ] Yes
[x] No
```

## Pull Request Type
What kind of change does this Pull Request introduce?

<!-- Please check the one that applies to this PR using "x". -->
```
[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:
```

Author: bindsi

samples/quickstarts/mqtt-client.yaml

@@ -1,4 +1,11 @@
 # Important: do not use in production environments
+# Create a service account
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: mqtt-client
+  namespace: azure-iot-operations
+---
 # Creates a pod with mosquitto-clients and mqttui utilities in your cluster
 apiVersion: v1
 kind: Pod
@@ -39,4 +46,4 @@ spec:
           expirationSeconds: 86400
   - name: trust-bundle
     configMap:
-      name: azure-iot-operations-aio-ca-trust-bundle # Default root CA cert
+      name: azure-iot-operations-aio-ca-trust-bundle # Default root CA cert