Add Outline API client library foundation

Implements a first vertical slice of the Outline RPC API (auth, documents,
collections, users) covering the structural patterns used across the whole
API: single object, paginated list, tree, create/update, delete, search.
Chosen as the basis for future CLI and GUI clients built on top of this crate.

- Handwritten client (not codegen) against the vendored OpenAPI spec, since
  Outline's API is uniformly POST /api/<resource>.<action> with JSON bodies
  and inline/anonymous schemas that generators handle poorly
- Async (reqwest + tokio) Client, cheaply cloneable, no &mut self methods
- thiserror-based Error with ErrorKind classification and boxed API error
  context; forward-compatible models (unknown fields ignored, unknown
  string-enum values preserved via a catch-all variant) since the API is
  unversioned and self-hosted instances vary in age
- Pagination via Page<T>/Paginator with next_page/collect_all/into_stream
- wiremock-based test suite plus a spec-coverage test guarding against typos
  in RPC method names

tests/fixtures/document.json

@@ -0,0 +1,27 @@
+{
+  "id": "5c3fa3dd-eb47-4239-8a5f-de5b5d6bf6e2",
+  "collectionId": "9d3d1b2e-3c9a-4c67-8a2a-7f5d3f0e9c11",
+  "parentDocumentId": null,
+  "title": "Welcome to Acme Inc",
+  "fullWidth": false,
+  "icon": "🎉",
+  "color": null,
+  "text": "# Welcome\n\nThis is the text of the document.",
+  "url": "/doc/welcome-to-acme-inc-hDYep1TPAM",
+  "urlId": "hDYep1TPAM",
+  "collaboratorIds": ["5c3fa3dd-eb47-4239-8a5f-de5b5d6bf6e2"],
+  "tasks": { "completed": 1, "total": 4 },
+  "revision": 12,
+  "createdAt": "2024-01-15T10:00:00.000Z",
+  "createdBy": {
+    "id": "5c3fa3dd-eb47-4239-8a5f-de5b5d6bf6e2",
+    "name": "Jane Doe",
+    "email": "jane@example.com",
+    "role": "admin",
+    "isSuspended": false
+  },
+  "updatedAt": "2024-02-20T09:30:00.000Z",
+  "publishedAt": "2024-01-15T11:00:00.000Z",
+  "archivedAt": null,
+  "deletedAt": null
+}