Add headers support to HTTP transport schema

Added optional 'headers' field to HTTP transport definition. Values support
${input:ID} placeholders for injecting user-provided values into request headers.
Enables Bearer token auth and other header-based authentication patterns.

https://claude.ai/code/session_013AxurW8hPAfnUNvik354Xx

Author: claude

schemas/server-definition.schema.json

@@ -167,6 +167,11 @@
               "format": "uri",
               "description": "HTTP endpoint URL (supports Streamable HTTP)"
             },
+            "headers": {
+              "type": "object",
+              "additionalProperties": { "type": "string" },
+              "description": "HTTP headers to include with requests. Values support ${input:ID} placeholders for user-provided values."
+            },
             "metadata": {
               "$ref": "#/$defs/metadata"
             }