feat(provider): add custom HTTP headers support for remote providers

Some subscription panels (e.g. Remnawave with incy/happ integration)
require device-identification headers to return the real server list
instead of a dummy config. Add a 'headers' field to the remote provider
options so users can attach arbitrary HTTP headers to subscription
fetch requests.

Signed-off-by: Arsolitt <arsolitt@gmail.com>

option/provider.go

@@ -55,6 +55,7 @@ type ProviderLocalOptions struct {
 type ProviderRemoteOptions struct {
 	URL            string             `json:"url"`
 	UserAgent      string             `json:"user_agent,omitempty"`
+	Headers        badoption.HTTPHeader   `json:"headers,omitempty"`
 	DownloadDetour string             `json:"download_detour,omitempty"`
 	UpdateInterval badoption.Duration `json:"update_interval,omitempty"`