add option to create a service account and specify a default user id in securityContext

[AndreZiviani]

I had two problems trying to use avhq in AWS EKS:

• Lack of ServiceAccount in the helm chart
• The container was running with the akhq user (uid=1000 gid=1000) but it wasn't specified in the securityContext so AWS created the token file with strict permissions (only the root user could read it)

This PR adds an option to automatically create the ServiceAccount and specify some options in the securitContext by default

[tchiotludo]

Can you use the standard way to handle service account for a better readability please ?

serviceAccountName: {{ include "bla.serviceAccountName" . }}

{{/*
Create the name of the service account to use
*/}}
{{- define "bla.serviceAccountName" -}}
{{- if .Values.serviceAccount.create }}
{{- default (include "bla.fullname" .) .Values.serviceAccount.name }}
{{- else }}
{{- default "default" .Values.serviceAccount.name }}
{{- end }}
{{- end }}

if the user input serviceAccountName & serviceAccountCreate the yaml will be invalid

[AndreZiviani]

@tchiotludo can you please review the changes?