feat: support passing resource metadata

[liady]

This PR adds the option to pass resource metadata over the _meta key. It allows passing mcp-ui specific metadata that can be used on the client side. See discussion in #49.

Server side

When using createUIResource() from the server SDK, you can now pass uiMetadata, metadata and resourceProps.

1. metadata will be translated to the standard _meta property on the resource.
2. uiMetadata is for mcp-ui specific configuration. The keys will be prefixed with mcpui.dev/ui-. There is a closed list:
   1. preferred-frame-size - for the resource to define its preferred initial frame size
   2. initial-render-data - for the resource to define an object that should be passed to it when rendering the iframe on the page
3. resourceProps will be spread and added to the resource definition, this allows adding additional spec-supported props (such as annotations).

createUIResource({
  uri: 'ui://user-form/1',
  content: {
    type: 'externalUrl',
    iframeUrl: 'https://yourapp.com'
  },
  encoding: 'text',

  // mcp-ui specific and typed metadata
  // these will be prefixed `mcpui.dev/ui-` and will be picked up by `UIResourceRenderer`
  uiMetadata: {
    // preferred initial frame size
    'preferred-frame-size': ['300px', '400px'],
    // initial data that will be sent back to the iframe on render
    'initial-render-data': {
      someKey: 'Some data'
    },
  },

  // these will be put as-is under the resource's `_meta` key, can be also read by `getResourceMetadata()`
  metadata: {
    'arbitrary-metadata': 'value',
    //...
  },

  // these will be passed as-is to the resource
  resourceProps: {
    annotations: {
      audience: 'user',
      priority: 0.8,
      //... etc
    }
  },
});

Renderer

On the client side, the <UIResourceRenderer /> can pick up the reserved mcp-ui keys from the metadata:

1. preferred-frame-size will be used as the initial frame size
2. initial-render-data will be merged with iframeRenderData prop (if exists) and will be passed using the ui-lifecycle-iframe-render-data mechanism

Utility functions

1. getResourceMetadata(resource) - returns the resource's _meta content
2. getUIResourceMetadata(resource) - returns only the mcp-ui keys from the resource _meta content

[liady]

Also check out @blittle proposal for secured data passing to iframes that can complement this PR
https://gist.github.com/blittle/247d50161e3ddb6bd6fa24a1c7c1f10f

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