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

main.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"log"
+
+	"github.com/hashicorp/terraform-plugin-framework/providerserver"
+
+	"git.unkin.net/unkin/terraform-provider-litellmvaultsecret/internal/provider"
+)
+
+var version = "0.0.1"
+
+func main() {
+	var debug bool
+	flag.BoolVar(&debug, "debug", false, "enable debug mode")
+	flag.Parse()
+
+	opts := providerserver.ServeOpts{
+		Address: "git.unkin.net/unkin/litellmvaultsecret",
+		Debug:   debug,
+	}
+
+	if err := providerserver.Serve(context.Background(), provider.New(version), opts); err != nil {
+		log.Fatal(err)
+	}
+}