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/litellm/config.yaml

@@ -0,0 +1,22 @@
+model_list:
+  - model_name: gpt-3.5-turbo
+    litellm_params:
+      model: openai/gpt-3.5-turbo
+      api_key: sk-fake-openai-key
+      mock_response: "hello from the mock model"
+  - model_name: gpt-4
+    litellm_params:
+      model: openai/gpt-4
+      api_key: sk-fake-openai-key
+      mock_response: "hello from the mock model"
+  - model_name: claude-3-sonnet
+    litellm_params:
+      model: anthropic/claude-3-sonnet
+      api_key: sk-fake-anthropic-key
+      mock_response: "hello from the mock model"
+
+general_settings:
+  master_key: sk-master-e2e-1234
+
+litellm_settings:
+  drop_params: true