Initial terraform-provider-ranchervaultsecret scaffold

Terraform provider (plugin-framework) for the vault-plugin-secrets-rancher
secrets engine, modeled on terraform-provider-litellmvaultsecret.

Resources:
- rancher_secret_backend: mount the engine + write config (rancher_url, ca_cert,
  tls_skip_verify, request_timeout_seconds).
- rancher_secret_backend_service_account: seed an auto-rotated Rancher token
  (write-only token; token_ttl / rotation_period; computed token_name,
  last_rotated).
- rancher_secret_backend_role: minting role (service_account, cluster_name,
  ttl, max_ttl, description).

Source address git.unkin.net/unkin/ranchervaultsecret, resources prefixed
rancher_. Ports the litellm Woodpecker terraform-registry release + nfpm-less
zip packaging, examples, and a provider e2e (Vault + mock Rancher from the
sibling plugin repo). Unit tests cover the coercion/import-ID helpers.

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-ranchervaultsecret/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/ranchervaultsecret",
+		Debug:   debug,
+	}
+
+	if err := providerserver.Serve(context.Background(), provider.New(version), opts); err != nil {
+		log.Fatal(err)
+	}
+}