Support query attributes

[bgrainger]

MySQL Server 8.0.23 introduces a new feature called "query attributes": Work Item, Documentation.

These are a list of name/value pairs that are sent in the COM_QUERY packet (before the query SQL).

One API possibility would be adding an MySqlParameterCollection MySqlCommand.Attributes { get; } property. This could be lazily-initialised. If non-null and non-empty, the attributes would be serialised into the query packet.

It may be useful to expose a bool MySqlConnection.SupportsQueryAttributes { get; } property so clients can know if this feature is supported. If it's not, setting the Attributes might throw an exception.

[bgrainger]

Connector/NET has added a MySqlCommand.Attributes collection in 8.0.26: https://dev.mysql.com/doc/connector-net/en/connector-net-programming-mysqlcommand.html#connector-net-programming-mysqlcommand-query-attributes

Add would be a better method name than SetAttribute, as that would allow collection initialisation syntax to be used.

[bgrainger]

If it's not, setting the Attributes might throw an exception.

In Connector/NET, "If you send query attribute metadata to a server that does not support query attributes, the attempt is logged by the connector but no error is emitted."

[bgrainger]

Added in 2.1.0.