fix: also detect sub classes of the SpannerDialect

[olavloite]

This registers the Spanner-specific DDL generation classes, also when a custom subclass of the com.google.cloud.spanner.hibernate.SpannerDialect is used. It also fixes a couple of minor issues with the DDL generation of this dialect:

1. Constraints are now dropped before tables are dropped.
2. Indexes on tables were already automatically dropped, but this PR adds an if exists clause to those drop statements to prevent potential failures if the index had already been dropped.