Support passing attributes with AddEvent

[rnburn]

Builds on top of #59

Allows attributes to be passed without copying from convertible key-value containers.

Examples of how it can be used

std::map<std::string, std::string> attributes1;
span->AddEvent("abc", attributes1);

std::vector<std::pair<std::string, int>> attributes2;
span->AddEvent("abc", attributes2);

span->AddEvent("abc", {{"a", 1}, {"b", "2"}, {"c", 3.0}});

[pyohannes]

I wonder about moving this from trace to common? We will need the same functionality for metrics labels and, I assume, also for logs at some point.

[rnburn]

Done

[pyohannes]

There's also this discussion going on: open-telemetry/opentelemetry-specification#376.

I'm not sure how serious that is, but it seems the idea is getting traction and is actively discussed currently. Maybe @reiley knows how likely this is to come and whether we should consider it.