how i can extend diffTool with storedProcedures option? I would like to implement an extension, then expand PostgresDatabase with that feature.
You can extend the SnapshotGenerator classes, which are the database-specific classes for reading the current database state and outputting it in a standard format that can be used by the diff tool. You extend them by creating subclasses and overriding the getPriority() method, like other extensions (liquibase.org/extensions).
The trouble will be that the main diff tool does not know to look for stored procedures in the generated snapshot, however, so you will need to make some changes to the core liquibase library to support that. The source is now on github.com/liquibase/liquibase if you want to look at what that would take.