feat(core): add anthropic provider (#11911)

Close [AI-40](https://linear.app/affine-design/issue/AI-40).

### What changed?
- Add anthropic provider.

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Added support for Anthropic as a new provider option in Copilot, enabling integration with Anthropic's text generation models.
  - Users can now configure Anthropic provider settings, including API key management, through the admin interface.

- **Chores**
  - Updated dependencies to include the Anthropic SDK for backend operations.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/package.json

@@ -27,6 +27,7 @@
   },
   "dependencies": {
     "@affine/server-native": "workspace:*",
+    "@ai-sdk/anthropic": "^1.2.10",
     "@ai-sdk/google": "^1.2.10",
     "@ai-sdk/openai": "^1.3.18",
     "@ai-sdk/perplexity": "^1.1.6",