[MCP] Allow specifying custom headers with streamable http servers (#5241)

This adds two new config fields to streamable http mcp servers:
`http_headers`: a map of key to value
`env_http_headers` a map of key to env var which will be resolved at
request time

All headers will be passed to all MCP requests to that server just like
authorization headers.

There is a test ensuring that headers are not passed to other servers.

Fixes #5180

codex-rs/cli/tests/mcp_add_remove.rs

@@ -112,9 +112,13 @@ async fn add_streamable_http_without_manual_token() -> Result<()> {
         McpServerTransportConfig::StreamableHttp {
             url,
             bearer_token_env_var,
+            http_headers,
+            env_http_headers,
         } => {
             assert_eq!(url, "https://example.com/mcp");
             assert!(bearer_token_env_var.is_none());
+            assert!(http_headers.is_none());
+            assert!(env_http_headers.is_none());
         }
         other => panic!("unexpected transport: {other:?}"),
     }
@@ -150,9 +154,13 @@ async fn add_streamable_http_with_custom_env_var() -> Result<()> {
         McpServerTransportConfig::StreamableHttp {
             url,
             bearer_token_env_var,
+            http_headers,
+            env_http_headers,
         } => {
             assert_eq!(url, "https://example.com/issues");
             assert_eq!(bearer_token_env_var.as_deref(), Some("GITHUB_TOKEN"));
+            assert!(http_headers.is_none());
+            assert!(env_http_headers.is_none());
         }
         other => panic!("unexpected transport: {other:?}"),
     }