ProfiledDbConnection: implement DisposeAsync

[bbarry]

https://docs.microsoft.com/en-us/dotnet/api/system.data.common.dbconnection.disposeasync?view=netcore-3.1

relates to dotnet/runtime#28596

ProfiledDbConnection should implement DisposeAsync by delegating to the wrapped connection's version to gain async disposal on connections that support it.

[NickCraver]

Are any providers using this today? Currently, MiniProfiler.Shared targets net461 and netstandard2.0, we'd need to add a netstandard2.1 build to expose this API, to trying to access value here :)

[bbarry]

NpgSql 4.1.0 and MySqlConnector 0.57.0 currently support it.

There are open issues for Microsoft.Data.SqlClient and AdoNetCore.AseClient (Sybase/SAP doesn't have a .NET Core driver yet at all) and I have no idea about any official Oracle stuff.

I don't think this should be a high priority or anything, just noted on a list of things to do eventually.