Add client connection attributes

[bgrainger]

Split from #288.

When connecting to the server, specify ProtocolCapabilities.ConnectAttributes and send some of the attributes documented here; at a minimum _client_name (= MySqlConnector) and _client_version (= current assembly version).

If they can be determined quickly, consider also sending _os, _pid and _platform.

[bgrainger]

Note that all these properties could be computed once, stored in a static buffer, then replayed on every connection.

[bgrainger]

Added in 0.23.0.