Allow passing custom headers for requests

[acoutts]

Required to use platform-specific API keys where you must provide the following headers:

    final headers = <String, String>{
      if (defaultTargetPlatform == TargetPlatform.iOS)
        'x-ios-bundle-identifier': Env().bundleIdentifier, // com.my.app
      if (defaultTargetPlatform == TargetPlatform.android) ...{
        'x-android-package': Env().bundleIdentifier, // com.my.app
        'x-android-cert': Env().ANDROID_SIGNING_KEY_SHA1, // signing key used to build/publish the app
      }
    };

[acoutts]

@Vitor-Bukovitz @bazrafkan @DiegoArredondo @kuroshmondora any chance to review this please? Thank you.