fix: Add missing vault_skip_tls_verify variable declaration

Added vault_skip_tls_verify variable to fix Terraform validation error.
This variable is referenced in provider.tf but was not declared in
variables.tf.

Default value is false for security, can be set to true for
self-signed certificates in development/testing environments.

variables.tf

@@ -133,3 +133,9 @@ variable "secret_id" {
   type        = string
   sensitive   = true
 }
+
+variable "vault_skip_tls_verify" {
+  description = "Skip TLS verification for Vault (useful for self-signed certificates)"
+  type        = bool
+  default     = false
+}