fcl-db: sqldb: implemented TSQLConnection.GetSchemaNames
Added stSchemata to TSchemaType (Delphi has strange stUserNames instead)
At TSQLConnection level stSchemata defaults to 'select *  from INFORMATION_SCHEMA.SCHEMATA', which can be overriden in descendants.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@23921 3ad0048d-3df7-0310-abae-a5850022a9f2