Fix visibility of DevSupportHttpClient by extracting DevSupportRequestHeaders (#55739)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55739

Extract header management from the `internal` `DevSupportHttpClient` into a new
public `DevSupportRequestHeaders` singleton in `devsupport.interfaces`. This
allows external consumers (e.g. Expo Dev Launcher) to register custom HTTP
headers that will be applied to all React Native dev-support network traffic
without depending on internal APIs.

`DevSupportHttpClient` now delegates to `DevSupportRequestHeaders.allHeaders()`
instead of maintaining its own `ConcurrentHashMap`.

A separate `request_headers` Buck target is introduced to avoid a dependency
cycle (`bridge` → `inspector` → `interfaces` → `bridge`).

Changelog: [Android][Added] - Public DevSupportRequestHeaders API for registering custom dev-support HTTP headers

Reviewed By: cipolleschi

Differential Revision: D94354168

fbshipit-source-id: 72aa65b615fe4ef43feaab10ffea7526589a76fd