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/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
+}