Add terraform-provider-litellmvaultsecret implementation

Populate the repo with the Terraform/OpenTofu provider that manages the LiteLLM
dynamic secrets engine on Vault/OpenBao via the Vault API.

- Provider (VAULT_ADDR/VAULT_TOKEN) with resources litellmvaultsecret_secret_backend
  (mount + config) and litellmvaultsecret_secret_backend_role (models, max_budget,
  ttl/max_ttl in seconds, metadata)
- Unit tests against a mock Vault API
- End-to-end test: builds the sibling plugin, boots Vault + LiteLLM + Postgres,
  and runs a real terraform apply/destroy asserting key generation works
- Makefile, woodpecker CI (build/test/pre-commit), examples, README

test/docker-compose.yml

@@ -0,0 +1,57 @@
+# E2E stack for the provider: Postgres + LiteLLM + a Vault dev server with the
+# litellm plugin mounted. Bind mounts use ":z" for SELinux (Fedora/RHEL).
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: litellm
+      POSTGRES_PASSWORD: litellm
+      POSTGRES_DB: litellm
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U litellm"]
+      interval: 3s
+      timeout: 3s
+      retries: 20
+
+  litellm:
+    image: ghcr.io/berriai/litellm:main-stable
+    depends_on:
+      postgres:
+        condition: service_healthy
+    environment:
+      LITELLM_MASTER_KEY: sk-master-e2e-1234
+      DATABASE_URL: postgresql://litellm:litellm@postgres:5432/litellm
+      STORE_MODEL_IN_DB: "True"
+    command: ["--config", "/app/config.yaml", "--port", "4000"]
+    volumes:
+      - ./litellm/config.yaml:/app/config.yaml:ro,z
+    ports:
+      - "4000:4000"
+    healthcheck:
+      test: ["CMD", "python", "-c", "import urllib.request,sys; sys.exit(0 if urllib.request.urlopen('http://localhost:4000/health/liveliness').status==200 else 1)"]
+      interval: 5s
+      timeout: 5s
+      retries: 40
+
+  vault:
+    image: hashicorp/vault:1.18
+    depends_on:
+      litellm:
+        condition: service_healthy
+    cap_add:
+      - IPC_LOCK
+    environment:
+      VAULT_DEV_ROOT_TOKEN_ID: root
+      VAULT_ADDR: http://127.0.0.1:8200
+      VAULT_TOKEN: root
+    command: ["server", "-dev", "-dev-listen-address=0.0.0.0:8200", "-config=/vault/vault.hcl"]
+    volumes:
+      - ./plugins:/vault/plugins:ro,z
+      - ./vault/vault.hcl:/vault/vault.hcl:ro,z
+    ports:
+      - "8200:8200"
+    healthcheck:
+      test: ["CMD", "vault", "status", "-address=http://127.0.0.1:8200"]
+      interval: 3s
+      timeout: 3s
+      retries: 20

test/e2e/main.tf

@@ -0,0 +1,29 @@
+terraform {
+  required_providers {
+    litellmvaultsecret = {
+      source = "git.unkin.net/unkin/litellmvaultsecret"
+    }
+  }
+}
+
+provider "litellmvaultsecret" {
+  address = "http://127.0.0.1:8200"
+  token   = "root"
+}
+
+resource "litellmvaultsecret_secret_backend" "litellm" {
+  path        = "litellm"
+  description = "LiteLLM dynamic virtual keys (e2e)"
+  # Reachable from inside the vault container, where the plugin runs.
+  base_url   = "http://litellm:4000"
+  master_key = "sk-master-e2e-1234"
+}
+
+resource "litellmvaultsecret_secret_backend_role" "team_a" {
+  backend    = litellmvaultsecret_secret_backend.litellm.path
+  name       = "team-a"
+  models     = ["gpt-3.5-turbo"]
+  max_budget = 10
+  ttl        = 3600
+  max_ttl    = 86400
+}

test/litellm/config.yaml

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+model_list:
+  - model_name: gpt-3.5-turbo
+    litellm_params:
+      model: openai/gpt-3.5-turbo
+      api_key: sk-fake-openai-key
+      mock_response: "hello from the mock model"
+  - model_name: gpt-4
+    litellm_params:
+      model: openai/gpt-4
+      api_key: sk-fake-openai-key
+      mock_response: "hello from the mock model"
+
+general_settings:
+  master_key: sk-master-e2e-1234
+
+litellm_settings:
+  drop_params: true

test/vault/vault.hcl

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+# Combined with `-dev` so the dev server has a plugin_directory to register the
+# litellm plugin binary mounted from ./plugins.
+plugin_directory = "/vault/plugins"
+api_addr         = "http://127.0.0.1:8200"