Make it possible to supply meta data for discovery and resources

[thekid]

This PR adds low-level support for adding _meta to discovery and to resource contents, enabling us support MCP Apps, the first step for implementing #19

Example

use io\modelcontextprotocol\server\{Implementation, Tool};

#[Implementation]
class Greeting {

  /** Greeting card */
  #[Tool(meta: ['ui' => ['resourceUri' => 'ui://greeting/app']])]
  public function card() {
    return 'App launching...';
  }

  /** Greeting app */
  #[Resource('ui://greeting/app', 'text/html;profile=mcp-app')]
  public function app() {
    return [
      '_meta' => ['ui' => ['csp' => ['resourceDomains' => ['http://localhost:8080']]]],
      'text'  => <<<'HTML'
         ...
      HTML,
    ];
  }
}

[thekid]

FastMCP uses a meta field inside the decorator, e.g.:

@mcp.tool(meta={
    "ui":{"resourceUri": VIEW_URI},
    "ui/resourceUri": VIEW_URI, # legacy support
})

This would save us extra checks for a Meta annotation during discovery, and would change the code as follows:

- #[Tool, Meta(['ui' => ['resourceUri' => 'ui://greeting/app']])]
+ #[Tool(meta: ['ui' => ['resourceUri' => 'ui://greeting/app']])]

[thekid]

Released in https://github.com/xp-forge/mcp/releases/tag/v0.13.0