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
