bug core: fix http client destruction order

This fixes crashes on service shutdown after the "split `HttpClientCore` and `HttpClient`" PR.

Estimated crash root cause: `components::TestsuiteSupport` could (via `testsuite::HttpAllowedUrlsExtra`) hold onto `clients::http::ClientCore` past the destruction of `components::HttpClientCore`. This would lead to `clients::http::ClientCore` continuing to serve detached requests (via `easy` & `multi`) after the dependencies of `HttpClientCore` (including `clients::dns::Component`, `components::StatisticsStorage`, `components::DynamicConfig`, and a part of `components::TestsuiteSupport` fields) are destroyed, which caused use-after-free in HTTP client internals.
commit_hash:8fd7fa69bc78c0cce74aa0fc6194f6169e703ffe

Author: smorivan

core/include/userver/testsuite/http_allowed_urls_extra.hpp

@@ -16,12 +16,12 @@ namespace testsuite {
 
 class HttpAllowedUrlsExtra final {
 public:
-    void RegisterHttpClient(std::shared_ptr<clients::http::ClientCore> http_client);
+    void RegisterHttpClient(clients::http::ClientCore& http_client);
 
     void SetAllowedUrlsExtra(std::vector<std::string>&& urls);
 
 private:
-    std::shared_ptr<clients::http::ClientCore> http_client_{nullptr};
+    clients::http::ClientCore* http_client_{nullptr};
 };
 
 }  // namespace testsuite

core/src/clients/http/component_core.cpp

@@ -93,7 +93,7 @@ HttpClientCore::HttpClientCore(const ComponentConfig& component_config, const Co
         http_client_->SetTestsuiteConfig({prefixes, timeout});
 
         auto& testsuite = context.FindComponent<components::TestsuiteSupport>();
-        testsuite.GetHttpAllowedUrlsExtra().RegisterHttpClient(http_client_);
+        testsuite.GetHttpAllowedUrlsExtra().RegisterHttpClient(*http_client_);
     }
 
     clients::http::impl::Config bootstrap_config;

core/src/testsuite/http_allowed_urls_extra.cpp

@@ -8,9 +8,7 @@ USERVER_NAMESPACE_BEGIN
 
 namespace testsuite {
 
-void HttpAllowedUrlsExtra::RegisterHttpClient(std::shared_ptr<clients::http::ClientCore> http_client) {
-    http_client_ = std::move(http_client);
-}
+void HttpAllowedUrlsExtra::RegisterHttpClient(clients::http::ClientCore& http_client) { http_client_ = &http_client; }
 
 void HttpAllowedUrlsExtra::SetAllowedUrlsExtra(std::vector<std::string>&& urls) {
     if (http_client_) {