Allow Dialect to specify fallback SchemaManagementTool

[sebersole]

Currently, SchemaManagementToolInitiator allows overriding the SchemaManagementTool by configuration setting. It then falls back to instantiating the standard implementation.

For some dialects, it is beneficial / necessary to use a custom SchemaManagementTool.

For example, on a dialect for Spanner these DDL statements effectively need to be "batched" together. The standard SchemaManagementTool assumes that each statement is executed singly. Against Spanner, however, the tool would need to collect the statements and execute them after.

I propose we add the following:

class Dialect {
    ...

    @Incubating
    public SchemaManagementTool getFallbackSchemaManagementTool(
            Map<String, Object> configurationValues, 
            ServiceRegistryImplementor registry) {
                return new HibernateSchemaManagementTool();
    }
}

[sebersole]

Specifically for the Spanner part, this should be as simple as pulling-up and exposing HibernateSchemaManagementTool#buildGenerationTargets - Spanner's would handle TargetType#DATABASE specially by supplying a "collecting" GenerationTarget.

At least in gist; other minor changes are needed to support that of course.

[sebersole]

https://hibernate.atlassian.net/browse/HHH-15763