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

.woodpecker/pre-commit.yaml

@@ -0,0 +1,18 @@
+when:
+  - event: pull_request
+
+steps:
+  - name: pre-commit
+    image: git.unkin.net/unkin/almalinux9-gobuilder:20260606
+    commands:
+      - uvx pre-commit run --all-files
+    backend_options:
+      kubernetes:
+        serviceAccountName: default
+        resources:
+          requests:
+            memory: 512Mi
+            cpu: 1
+          limits:
+            memory: 2Gi
+            cpu: 2