Rename resources to litellm_secret_* prefix

Use the shorter, cleaner litellm_ resource prefix instead of
litellmvaultsecret_. The provider source stays git.unkin.net/unkin/
litellmvaultsecret; only the resource prefix (provider Metadata TypeName)
changes to litellm, declared under the local name litellm in
required_providers — the same pattern google-beta uses to ship google_*.

- Provider TypeName litellmvaultsecret -> litellm
- Resources: litellm_secret_backend, litellm_secret_backend_role
- Update examples (dirs + contents), README, and the e2e terraform config

examples/resources/litellm_secret_backend/main.tf

@@ -0,0 +1,24 @@
+terraform {
+  required_providers {
+    litellm = {
+      source = "git.unkin.net/unkin/litellmvaultsecret"
+    }
+  }
+}
+
+provider "litellm" {
+  # address and token fall back to VAULT_ADDR / VAULT_TOKEN
+  address = "https://vault.example.com"
+}
+
+resource "litellm_secret_backend" "litellm" {
+  path        = "litellm"
+  description = "LiteLLM dynamic virtual keys"
+  base_url    = "http://litellm.litellm.svc:4000"
+  master_key  = var.litellm_master_key
+}
+
+variable "litellm_master_key" {
+  type      = string
+  sensitive = true
+}