Add LiteLLM dynamic secrets engine implementation

Populate the repo with the Vault/OpenBao dynamic secrets engine that mints
LiteLLM virtual keys scoped by model, spending limit, and lease TTL.

- Secrets backend: config, roles, creds paths and a revocable litellm_key type
- LiteLLM API client (generate/update/delete/info) with master-key auth
- Unit tests (mock LiteLLM) and a docker-compose e2e against both Vault and
  OpenBao proving the same binary works on each
- Makefile, woodpecker CI (build/test/pre-commit), pre-commit config

test/vault/vault.hcl

@@ -0,0 +1,4 @@
+# Combined with `-dev` at runtime; supplies the plugin_directory the dev server
+# would otherwise leave unset, so the plugin binary in ../dist can be registered.
+plugin_directory = "/vault/plugins"
+api_addr         = "http://127.0.0.1:8200"