Add missing assignment operators

Summary:
related to T13767
- add missing assignment operators
- marked single argument constructors as explicit where needed
- add even more missing includes

Reviewers: ivica

Reviewed By: ivica

Subscribers: iljazovic, miljen

Differential Revision: https://repo.mireo.local/D30813

include/async_mqtt5/impl/client_service.hpp

@@ -133,6 +133,7 @@ class stream_context<
 	mqtt_ctx _mqtt_context;
 public:
 	explicit stream_context(std::monostate) {}
+
 	stream_context(const stream_context& other) :
 		_mqtt_context(other._mqtt_context)
 	{}
@@ -271,7 +272,7 @@ private:
 
 public:
 
-	client_service(
+	explicit client_service(
 		const executor_type& ex,
 		tls_context_type tls_context = {}
 	) :