Custom expirations for token flow OAuth

In order to take advantage of this change, you would extend Doorkeeper configuration with the custom_access_token_expiration lambda that takes a Doorkeeper application as an input. In our application, it would work as follows:

```ruby
Doorkeeper.configure do
  custom_access_token_expiration do |app|
    app.api_app.token_expiration
  end
end
```

@aq1018 @hlfcoding