[repository] remove unused check_session method and its test Related to #1381328.

[migration/test] avoid test interdependency by removing fakecustomtype cube There is still a failing test afterward but it doesn't seem related to changes in this series.

[migration/test] use numeric(precision, scale) instead of geometry The latter requires the postgis extension which is overkill for this test.

[server, hooks] allow callable in dbh.TYPE_MAPPING This is necessary for cubes introducing new custom final types whose generated sql may depend on some extra configuration, avoiding ugly-buggy hack as done currently by the postgis cube. Closes #7569998

[migration] consider extra_props in rdef created during attribute creation hook else we may miss important data such as type parametrization (see postgis.Geometry for instance). Related to #7569998

[migration] only define variable if needed

[migration] handle non-existing attribute type when adding an entity type The final type needs to be added to the schema first. Related to #7569998.

[migration] handle the case where new final entity type is not yet in the schema This may occur on new cube addition. Related to #7569998.

[migration] config.add_cubes should ensure site_cubicweb of new cubes is called Related to #7569998

[migration] test addition of a cube providing a custom final type This does not work yet. Related to #7569998