Multi-tenant class not found if defined inside a module

The following code works as expected and the call to `multi_tenant :tenant` automatically calls `belongs_to :tenant` and defines both `tenant` and `tenant=` methods.

```ruby
class Tenant < ApplicationRecord; end

class Foo < ApplicationRecord
multi_tenant :tenant
end
```

However, if both classes are inside a module

```ruby
module MyModule
class Tenant < ApplicationRecord; end

class Foo < ApplicationRecord
multi_tenant :tenant
end
end
```

the `multi_tenant :tenant` call does not automatically call `belongs_to :tenant` or define `tenant` and `tenant=` methods.
This is due to the implementation of `tenant_klass_defined?`

```ruby
def self.tenant_klass_defined?(tenant_name)
!!tenant_name.to_s.classify.safe_constantize
end
```

so only the top-level constant `::Tenant` is expected to be the tenant class for tenant name `:tenant`, `::MyModule::Tenant` is not considered.

I thought about copying Rails' implementation of `compute_type`

https://github.com/rails/rails/blob/6ef39975d60cc9dafd1728c49e394dad11d12327/activerecord/lib/active_record/inheritance.rb#L224-L235

that in our example would search for the following 3 classes:

`::MyModule::Foo::Tenant`, `::MyModule::Tenant`, `::Tenant`

in that order.

Another solution would be to pass `class_name` just like we do in Rails.

`multi_tenant :tenant, class_name: "::MyModule::Tenant"`

What do you guys think?

I'm willing to implement this if we can agree on a solution.