Support appending custom connection string options to some resources

[davidfowl]

Today connection strings are owned by the resource type but sometimes they need to be appended to set other options (MARS in sql for example). We should allow apps to append to the connection string expression in WithReference or at the resource level (where it makes sense).

Today this is doable with manual logic but it might need to be more first class.

[Kralizek]

Another use case besides enabling MARS is enabling special logging on PostgreSQL (see)