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 (git.unkin.net/unkin/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/modules/litellm_secret_backend/terraform.tf

@@ -6,7 +6,7 @@ terraform {
       version = "5.6.0"
     }
     litellm = {
-      source  = "git.unkin.net/unkin/litellmvaultsecret"
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/litellmvaultsecret"
       version = "0.1.0"
     }
   }

modules/vault_cluster/modules/litellm_secret_backend_role/terraform.tf

@@ -2,7 +2,7 @@ terraform {
   required_version = ">= 1.10"
   required_providers {
     litellm = {
-      source  = "git.unkin.net/unkin/litellmvaultsecret"
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/litellmvaultsecret"
       version = "0.1.0"
     }
   }