Now that we have implemented granular connection swapping in #40370
we need a new API that will allow connections to multiple databases. The
reason we need this API is it will prevent deep nesting in cases where
we know that we want 3 of our 5 databases to connect to reading and
leave the rest on writing.

With this API, instead of writing:

```ruby
AnimalsRecord.connected_to(role: :reading) do
  MealsRecord.connected_to(role: :reading) do
    Dog.first # read from animals replica
    Dinner.first # read from meals replica
    Person.first # read from primary writer
  end
end
```

This API would allow you to write:

```ruby
ActiveRecord::Base.connected_to_many([AnimalsRecord, MealsRecord], role: :reading) do
  Dog.first # read from animals replica
  Dinner.first # read from meals replica
  Person.first # read from primary writer
end
```

This would come in especially handy for deeper nesting past 2 databases.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>