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

examples/resources/litellmvaultsecret_secret_backend_role/main.tf

@@ -0,0 +1,14 @@
+resource "litellmvaultsecret_secret_backend_role" "team_a" {
+  backend = litellmvaultsecret_secret_backend.litellm.path
+  name    = "team-a"
+
+  models     = ["gpt-3.5-turbo", "gpt-4"]
+  max_budget = 50
+  ttl        = 3600  # seconds (1h)
+  max_ttl    = 86400 # seconds (24h)
+
+  metadata = {
+    team = "a"
+    env  = "prod"
+  }
+}