Provide existing secret for restic backups

[ldaneliukas]

Use Case

In order to use restic for backups, you need to commit plain-text secrets into your VCS since the chart does not allow using an existing secret for these credentials.

Describe the Solution You Would Like

Use of existing secret for restic S3 credentials, similarly to the way that PostgreSQL secrets are already handled.

Describe Alternatives You've Considered

• Using plain text secrets, but that's not really an alternative.
• Injecting require Helm values from CI/CD (e.g. from GitHub Actions secrets) which works, but it'd be nicer to be able to use a pre-existing secret as it can be pre-created by Vault (or similar) in most Kubernetes setups.

Additional Context

I can create a PR for this.

[Xtigyro]

Same here - please create a PR if you can.
CC: @davidphay