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/frontend/admin/src/config.json

@@ -233,6 +233,10 @@
       "type": "Object",
       "desc": "The config for the perplexity provider."
     },
+    "providers.anthropic": {
+      "type": "Object",
+      "desc": "The config for the anthropic provider."
+    },
     "unsplash": {
       "type": "Object",
       "desc": "The config for the unsplash key."