Move HTTP service to public for plugin use (#27284)

* Move httpservice for use in plugins

* Adapt httpservice for plugin use

* Fix lint

server/public/shared/httpservice/transport.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package httpservice
+
+import (
+	"net/http"
+)
+
+// MattermostTransport is an implementation of http.RoundTripper that ensures each request contains a custom user agent
+// string to indicate that the request is coming from a Mattermost instance.
+type MattermostTransport struct {
+	// Transport is the underlying http.RoundTripper that is actually used to make the request
+	Transport http.RoundTripper
+}
+
+func (t *MattermostTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	req.Header.Set("User-Agent", defaultUserAgent)
+
+	return t.Transport.RoundTrip(req)
+}