Specify varchar length for primary keys

While the user can define a column as simply “varchar”, that column cannot be used as primary key.

In case the user wants to use varchar as primary key, the user has to to specify its length (i.e., `varchar[80]`)

** solution **
-> We should make possible for the user to define `varchar` columns as primary keys without specifying the length.

No files or tests are named. Start by locating the SQL schema handling for varchar columns and the validation or creation path for primary keys. Done means an unspecified-length varchar column can be used as a primary key, while existing length-specific varchar behavior remains intact.

Written by the indexing model from the issue text.