Consider adding counter cache functionality that uses triggers

Nobody has claimed this yet.

`Triggered` by #337.

This may be something that should be its own shard. I've considered creating a shard for this, but I thought it would be good to discuss it here first.

I have a couple counter caches in my app that are implemented with triggers. The migrations are definitely a little hairy and it would be nice to be able to just say something like:

```crystal
def migrate
alter table_for(Comments) do
add_counter_cache like_count : Int64, count_table: :likes, foreign_key: comment_id
end
end

def rollback
alter table_for(Comments) do
remove_counter_cache :like_count, count_table: :likes, foreign_key: comment_id
end
end
```

Here's some actual code to create a counter cache trigger. There's a lot of boilerplate in there:

```crystal
def migrate
execute <<-SQL
CREATE FUNCTION maintain_campaign_call_count_trg() RETURNS TRIGGER AS
$$
BEGIN
IF TG_OP IN ('UPDATE', 'DELETE') THEN
UPDATE campaigns SET call_count = call_count - 1 WHERE id = old.campaign_id;
END IF;
IF TG_OP IN ('INSERT', 'UPDATE') THEN
UPDATE campaigns SET call_count = call_count + 1 WHERE id = new.campaign_id;
END IF;
RETURN NULL;
END
$$
LANGUAGE plpgsql;
SQL
execute <<-SQL
CREATE TRIGGER maintain_campaign_call_count
AFTER INSERT OR UPDATE OF campaign_id OR DELETE ON calls
FOR EACH ROW
EXECUTE PROCEDURE maintain_campaign_call_count_trg();
SQL

execute <<-SQL
CREATE FUNCTION maintain_campaign_call_count_after_truncate_trg() RETURNS TRIGGER AS
$$
BEGIN
UPDATE campaigns SET call_count = 0;
RETURN NULL;
END
$$
LANGUAGE plpgsql;
SQL
execute <<-SQL
CREATE TRIGGER maintain_campaign_call_count_after_truncate
AFTER TRUNCATE ON calls
EXECUTE PROCEDURE maintain_campaign_call_count_after_truncate_trg();
SQL
end

def rollback
execute "DROP TRIGGER maintain_campaign_call_count ON calls;"
execute "DROP FUNCTION maintain_campaign_call_count_trg();"

execute "DROP TRIGGER maintain_campaign_call_count_after_truncate ON calls;"
execute "DROP FUNCTION maintain_campaign_call_count_after_truncate_trg();"
end
```

The issue names migration entry points using alter table_for, add_counter_cache, remove_counter_cache, and raw execute SQL, but no repository files or tests. Start by locating those migration APIs and reviewing the PostgreSQL trigger examples; done would require an agreed scope and verified migration and rollback behavior.

Written by the indexing model from the issue text.