Idea: Add RowDescription to SimpleQueryMessage

[rigby-dane]

Currently when processing the SimpleQueryMessage, if there are no DataRows coming from postgres the column data is inaccessible since the data is embedded in the SimpleQueryRow. In particular, this makes it difficult if you're displaying user submitted or dynamic query results where being able to determine the columns from the query after it's been sent to the server would be useful. Adding an enum value of RowDescription(Arc<[SimpleColumn]>) would alleviate this since you could separately process the columns even without a DataRow being returned.

[sfackler]

Seems reasonable to me. SimpleQueryMessage is already marked non_exhaustive so this should be a backwards compatible change.