feat: manage litellm secrets engine via terraform-provider-litellmvaultsecret

The vault-plugin-secrets-litellm engine mints LiteLLM virtual keys and is
now registered in Vault, but nothing declared its mount/config or roles in
this repo. Wire in the companion litellm provider so the mount is managed
as code alongside the other secret backends.

- Add litellm_secret_backend module: mounts the engine and writes its
  config (base_url, request_timeout_seconds); reads master_key from KV
- Add litellm_secret_backend_role module: manages roles (models,
  max_budget, key_alias_prefix, ttl/max_ttl seconds, metadata)
- Register both modules in vault_cluster main.tf and variables.tf
- Discover litellm_secret_backend[_role] YAML in config.hcl and pass them
  through terragrunt inputs
- Declare the litellm provider (litellmvaultsecret) and
  a provider block in the generated root backend.tf
- Add example config for the litellm mount and a sample role

modules/vault_cluster/variables.tf

@@ -289,6 +289,32 @@ variable "kubernetes_secret_backend_role" {
   default = {}
 }
 
+variable "litellm_secret_backend" {
+  description = "Map of LiteLLM secret engines to create (mount + config). The master key is read from KV"
+  type = map(object({
+    plugin                  = optional(string, "vault-plugin-secrets-litellm")
+    description             = optional(string)
+    base_url                = string
+    request_timeout_seconds = optional(number, 30)
+  }))
+  default = {}
+}
+
+variable "litellm_secret_backend_role" {
+  description = "Map of LiteLLM roles to create"
+  type = map(object({
+    name             = string
+    backend          = string
+    models           = optional(list(string))
+    max_budget       = optional(number)
+    key_alias_prefix = optional(string)
+    ttl              = optional(number)
+    max_ttl          = optional(number)
+    metadata         = optional(map(string))
+  }))
+  default = {}
+}
+
 variable "policy_auth_map" {
   description = "Map of auth mounts -> auth roles -> policy names"
   type        = map(map(list(string)))