Allow serialized ID to be overwritten for belongs-to relationships

If the `id` attribute for a class isn't taken directly from the object when
serializing it, it may be desirible for other classes that serialize a
relationship with that class to overwrite the relationship IDs they serialize.

For example, suppose we have:

```(ruby)
class Repo < Model
  attributes :id, :github_id, :name
  associations :configs
end

class Config < Model
  attributes :id
  belongs_to :repo
end

class RepoSerializer < ActiveModel::Serializer
  attributes :id, :name

  has_many :update_configs

  def id
    object.github_id
  end
end

class ConfigSerializer < ActiveModel::Serializer
  attributes :id
  belongs_to :repo
end
```

In the above example, serializing a list of `Repo`s will give the `github_id`
for each one, but serializing a `Config` will give the `id` for its parent repo.

Ideally AMS would inspect the `RepoSerializer` when serializing the `Config`,
and realise it can't just output the foreign key. Unfortunately, getting the
serialization class for the child repo currently requires loading the record
(via evaluating `lazy_assocation`), and loses the performance benefit of the
existing `belongs_to?` path. Instead, I've opted to use
`read_attribute_for_serialization` instead of `object.send` to fetch the
serialized foreign key. This allows the serialized relationship ID to be
overwritten using

```(ruby)
class ConfigSerializer < ActiveModel::Serializer
  ...

  def repo_id
    object.repo.github_id
  end
end
```

CHANGELOG.md

@@ -7,6 +7,7 @@ Breaking changes:
 Features:
 
 - [#2021](https://github.com/rails-api/active_model_serializers/pull/2021) ActiveModelSerializers::Model#attributes. Originally in [#1982](https://github.com/rails-api/active_model_serializers/pull/1982). (@bf4)
+- [#2130](https://github.com/rails-api/active_model_serializers/pull/2130) Allow serialized ID to be overwritten for belongs-to relationships. (@greysteil)
 
 Fixes: