Add terraform-provider-litellmvaultsecret implementation

Populate the repo with the Terraform/OpenTofu provider that manages the LiteLLM
dynamic secrets engine on Vault/OpenBao via the Vault API.

- Provider (VAULT_ADDR/VAULT_TOKEN) with resources litellmvaultsecret_secret_backend
  (mount + config) and litellmvaultsecret_secret_backend_role (models, max_budget,
  ttl/max_ttl in seconds, metadata)
- Unit tests against a mock Vault API
- End-to-end test: builds the sibling plugin, boots Vault + LiteLLM + Postgres,
  and runs a real terraform apply/destroy asserting key generation works
- Makefile, woodpecker CI (build/test/pre-commit), examples, README

test/litellm/config.yaml

@@ -0,0 +1,17 @@
+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"
+
+general_settings:
+  master_key: sk-master-e2e-1234
+
+litellm_settings:
+  drop_params: true