Introspect default constraint name

[morozov]

The current approach to managing default column values on SQL Server has a few significant downsides:

1. The schema manager manages the schema with the assumption that the foreign key constraints are named in a certain way:

   dbal/src/Platforms/SQLServerPlatform.php

   Lines 1216 to 1219 in 171759e

   	private function generateDefaultConstraintName(string $table, string $column): string
   	{
   	return 'DF_' . $this->generateIdentifierName($table) . '_' . $this->generateIdentifierName($column);
   	}

   
   It makes it unable to manage the schema that was not created by the DBAL.
2. In order to maintain the naming of the default constraints with the above convention, the schema manager has to rename the affected default constraints if the table or the column that it belongs to is renamed:
   1. Renaming column:

      dbal/src/Platforms/SQLServerPlatform.php

      Line 463 in 171759e

      // Recreate default constraint with new column name if necessary (for future reference).

   2. Renaming table:

      dbal/src/Platforms/SQLServerPlatform.php

      Lines 492 to 499 in 171759e

      	/**
      	* Rename table's default constraints names
      	* to match the new table name.
      	* This is necessary to ensure that the default
      	* constraints can be referenced in future table
      	* alterations as the table name is encoded in
      	* default constraints' names.
      	*/

The proposed approach eliminates the dependency on the naming convention and instead introspects the default constraints.