DatabaseSchemaEditor: add a hook to run DML statements after the entire schema is created

[Maximiliano Robaina]

A nice option would be to have a hook to run DML statements after all migrations were applied and commited.

A uses case (for example in Firebird SQL) is to add a new field on already existing and populated table. Is not enough set and drop a default value, we need to update the new field with efective default value. It is not possible without commit the schema alteration.
An aproach could be to have a kind of hook to run sql statements (DML statement in this case) when the schema altereation finished (and commited). Of course, must bu runned in another transaction.

This feature request is to better support of third-party database backends.

[Tim Graham]

Did you forgot about #26399 that you created or is this is a different issue?

[Maximiliano Robaina]

Tim,

It's a different issue. May be I need to write It better.
In this case is just for run a DML statement after all migrations were applied (and commited). About change of defaults values that I mencioned is just an uses case.