feat(spec): add icon field to MCP manifest spec

Add optional icon field to server metadata for displaying server icons in clients. Update JSON schema with URI format validation. Add documentation explaining icon should be square PNG/SVG with fallback behavior. Update examples with icon URLs

examples/github.json

@@ -10,6 +10,7 @@
     "homepage": "https://github.com",
     "repository": "https://github.com/anthropics/mcp-servers",
     "license": "MIT",
+    "icon": "https://github.githubassets.com/favicons/favicon.svg",
     "keywords": ["github", "git", "code", "issues", "pull-requests"]
   },
   "install": [