DacFx - only return public types as properties

If you have a TSqlObject like a `Column`, you can get the `Length` using:

```csharp
var length = column.GetProperty(Column.Length);
```

Length is an int of the actual length. If we wanted to know something more exotic such as the expression on a calculated column we should do:

```csharp
var expression = column.GetProperty(Column.Expression);
```

*but* SqlScriptProperty is an internal class so we can't use the typed version, instead we need to fall back to getting a `object`:

```csharp
var expression = column.GetProperty(Column.Expression);
```

It happens to be a string but we don't know at compile time what the type is or just always assume it ends up being a string.

We can't force a string as in:

```csharp
var expression = column.GetProperty(Column.Expression);
```

Because we get a cast failure (can't case SqlScriptProperty to string)

Please can you only return public classes when we use TSqlObject.GetProperty.

**If there is somewhere better to raise DacFx issues please say :) **

Start at the TSqlObject.GetProperty API and trace how Column.Length and Column.Expression are resolved, including the internal SqlScriptProperty type. Confirm the intended behavior for public and object-based access, then verify that callers can retrieve supported public types without exposing internal classes.

Written by the indexing model from the issue text.