[bitnami/Ghost] Correctly handle smtpExistingSecret value

[AverageMarcus]

Description of the change

Correctly handles setting either .Values.smtpPassword or .Values.smtpExistingSecret. Default smtp-password is now created in its own Secret if an existing is not provided.

Benefits

Fixes the need to provide a dummy smtpPassword value when using an external secret.

Possible drawbacks

The SMTP secret, if an existing Secret resource is not provided, is now in its own Secret resource rather than the one used for the Ghost auth credentials (to be more in-line with the database secret)

Applicable issues

• fixes [bitnami/ghost] smtpExistingSecret value is ignored #8916

Additional information

Checklist

• Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• All commits signed off and in agreement of Developer Certificate of Origin (DCO)