feat userver: allow creating custom implicit options

[trenin17]

trafficstars

Usage example:

class CustomImplicitOptions final : public userver::server::handlers::ImplicitOptions {
 public:
    CustomImplicitOptions(const userver::components::ComponentConfig& config,
                          const userver::components::ComponentContext& context,
                          bool is_monitor = false) : ImplicitOptions(config, context, is_monitor) {}

    std::string HandleRequestThrow(const userver::server::http::HttpRequest& request,
                                   userver::server::request::RequestContext& context) const override {
    request.GetHttpResponse().SetHeader(static_cast<std::string>("Access-Control-Allow-Origin"), "*");
    request.GetHttpResponse().SetHeader(static_cast<std::string>("Access-Control-Allow-Headers"), "*");

    return ImplicitOptions::HandleRequestThrow(request, context);
}
};

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[trenin17]

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