Extension point needed after applying migrations programmatically

[roji]

Original issue: npgsql/efcore.pg#292

tl;dr PostgreSQL has extensions, which can be added via the regular migration process. However, since extensions can add database types, the ADO layer must be instructed to reload types.

EFCore.PG does this if the migration is applied when first creating a database, since RelationalDatabaseCreator is public. However, for users applying migrations programmatically (context.Database.Migrate()) this would currently require extending Migrator, which is an internal type.

This is a non-critical issue - users can work around it by triggering type reloading themselves. Npgsql can also extend Migrator for now even though it's internal.

[bricelam]

Possibly covered by #24710

[AndriySvyryd]

This can also be used for seeding.

[AndriySvyryd]

The callback will be executed in a transaction, and it needs to be idempotent.