Fix get_columns_in_relation datatypes for UNICODE datatypes

[tkirschke]

Issue

UNICODE datatypes (nvarchar, nchar, sysname) need two bytes per character.
If a source column has the datatype nvarchar(4000), the sys.columns shows max_length = 8000, because it reflects the number of bytes required to store this value, see docs (here)[https://learn.microsoft.com/en-us/sql/relational-databases/system-catalog-views/sys-columns-transact-sql?view=sql-server-ver17].

Since this is used to return the datatypes, the wrong datatype nvarchar(8000) is returned, which would actually cause errors, at least in MSSQL.

What's changed

Instead of just forwarding sys.columns.max_length, I have added a CASE WHEN that checks for the datatype. If the datatype is a UNICODE datatype (nvarchar, nchar, sysname), the max_length is divided by 2, unless max_length is set to -1, because that value is used to indicate MAX.