Change copy constructor of endpoint to const &

[RicardoMDomingues]

Fix a compilation error when I use the websocketpp::client in a std::variant using libc++. lbstdc++ works fine.

Example of code that does not compile using libc++

#include <websocketpp/config/asio_no_tls_client.hpp>
#include <websocketpp/config/asio_client.hpp>
#include <websocketpp/client.hpp>
#include <variant>

class WebSocketClient
{
    using WsClient = websocketpp::client<websocketpp::config::asio_client>;
    using WssClient = websocketpp::client<websocketpp::config::asio_tls_client>;

    std::variant<WssClient, WsClient> websocketClient;
};

Compilation error:

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/variant:716:1: error: the parameter for this explicitly-defaulted copy constructor is const, but a member or base requires it to be non-const
_LIBCPP_VARIANT_UNION(_Trait::_Available, ~__union() {});
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/variant:699:5: note: expanded from macro '_LIBCPP_VARIANT_UNION'
    __union(const __union&) = default;                                         \
    ^

...
    
                                  ^
In file included from /Users/ricardo.domingues/my-project/src/WebSocketClient.cpp:6:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/include/c++/v1/variant:716:1: error: the parameter for this explicitly-defaulted copy constructor is const, but a member or base requires it to be non-const
_LIBCPP_VARIANT_UNION(_Trait::_Available, ~__union() {});

[Radrik5]

websocketpp::client should be wrapped in shared_ptr to avoid crashes in endpoint (fixed in #1041).