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

@@ -1,17 +1,17 @@
 terraform {
   required_providers {
-    litellmvaultsecret = {
+    litellm = {
       source = "git.unkin.net/unkin/litellmvaultsecret"
     }
   }
 }
 
-provider "litellmvaultsecret" {
+provider "litellm" {
   # address and token fall back to VAULT_ADDR / VAULT_TOKEN
   address = "https://vault.example.com"
 }
 
-resource "litellmvaultsecret_secret_backend" "litellm" {
+resource "litellm_secret_backend" "litellm" {
   path        = "litellm"
   description = "LiteLLM dynamic virtual keys"
   base_url    = "http://litellm.litellm.svc:4000"

examples/resources/litellm_secret_backend_role/main.tf

@@ -1,5 +1,5 @@
-resource "litellmvaultsecret_secret_backend_role" "team_a" {
-  backend = litellmvaultsecret_secret_backend.litellm.path
+resource "litellm_secret_backend_role" "team_a" {
+  backend = litellm_secret_backend.litellm.path
   name    = "team-a"
 
   models     = ["gpt-3.5-turbo", "gpt-4"]