Extend sqlexp.MsgNotice to capture SQL notice fields like state

[shueybubbles]

Is your feature request related to a problem? Please describe.
The driver discards state/class/number fields from tokenInfo.
sqlcmd needs access to those fields, per microsoft/go-sqlcmd#76
The driver should extend sqlexp.MsgNotice in a way that preserves the existing contract while provided the remaining fields to apps that know about them.

Describe the solution you'd like
Either:
Implement a new type that extends sqlexp.MsgNotice, and either:

1. Post both the mssql-specific message type and the MsgNotice struct and rely on the client to only use one of the two, or
2. Extend MsgNotice in a way that existing switch statements on the type will work for clients that don't know about the new type.