Add snippets with most frequently used migrations

[drednout]

Following frequently used code should be wrapped as snippets for sdbmigrate:

1. stored procedure
2. plain migration
3. sharded migration
4. alter table add uniq column
5. alter table add constraint
6. alter table add column boolean default True

See article and video:
https://www.youtube.com/watch?v=P3ctIoICkOc
https://habr.com/ru/company/yandex/blog/435880/

[drednout]

sdbmigrate 1.1.0 has generate action, which may be extended by new templates to generate snippets above.