[DBAL-122] Fix BLOB type mapping in SQL Server platform

This PR fixes mappings for database types `image` and `binary` in SQL Server which should be mapped to Doctrine's `BlobType`.
`image` is mapped to `VARBINARY(MAX)` in SQL Server internally. Mapping `binary` to `BlobType` is not 100% correct as it has a fixed length and is limited in its max size, but as long as we don't have a Doctrine type like `BinaryType`, this is definitly better than mapping it to `text` which is simply wrong.