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_role/main.tf

@@ -0,0 +1,14 @@
+resource "litellm_secret_backend_role" "team_a" {
+  backend = litellm_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"
+  }
+}