feat: Add a custom header to the SDK for chatting with the agent. (#12430)

### What problem does this PR solve?

As title.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

Co-authored-by: Liu An <asiro@qq.com>

agent/component/agent_with_tools.py

@@ -76,6 +76,8 @@ class AgentParam(LLMParam, ToolParamBase):
         self.mcp = []
         self.max_rounds = 5
         self.description = ""
+        self.custom_header = {}
+
 
 
 class Agent(LLM, ToolBase):
@@ -105,7 +107,8 @@ class Agent(LLM, ToolBase):
 
         for mcp in self._param.mcp:
             _, mcp_server = MCPServerService.get_by_id(mcp["mcp_id"])
-            tool_call_session = MCPToolCallSession(mcp_server, mcp_server.variables)
+            custom_header = self._param.custom_header
+            tool_call_session = MCPToolCallSession(mcp_server, mcp_server.variables, custom_header)
             for tnm, meta in mcp["tools"].items():
                 self.tool_meta.append(mcp_tool_metadata_to_openai_tool(meta))
                 self.tools[tnm] = tool_call_session