Feat #1732: Add helm_diff data source for plan-time release comparison

[dennismdejong]

Adds a new helm_diff data source that shows the diff between the currently deployed Helm release and proposed changes.

Usage

data "helm_diff" "example" {
  name       = "my-release"
  namespace  = "default"
  chart      = "stable/nginx"
  values     = [file("values.yaml")]
}

Output

• diff - Unified diff between current and proposed manifests
• current_manifest - Currently deployed release manifest
• proposed_manifest - Proposed manifest with the configured values