[Feature]: Ability to use existing secret for usernameAdminPassword

[anified]

Feature Description

Adding an optional solace.existingSecret value to helm chart for enabling usernameAdminPassword deployment with encryption.

Use Case

Do not want to save plain text password in repository for CI/CD deployment.

Proposed Solution

Wrap secret.yaml:
{{- if not .Values.solace.existingSecret }}

For solaceStatefulSet.yaml:
Change:
secretName: {{ template "solace.fullname" . }}-secrets
To:
secretName: {{ .Values.solace.existingSecret | default (printf "%s-secrets" (include "solace.fullname" .)) }}