Add RCTDevSupportHttpHeaders singleton (#55586)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55586

Add a thread-safe singleton that holds custom HTTP headers to be applied to all iOS devsupport network requests. This mirrors the Android `DevSupportHttpClient` from D93481539. Uses a GCD concurrent queue with barrier writes for reader-writer synchronization.

Changelog:
[Internal] [Changed] -

Reviewed By: cipolleschi

Differential Revision: D93490954

fbshipit-source-id: addb2f11025a046eedd0e296a7c398eef7211dc4