Add flag for excluding Rails default attributes from PHI wrapping

We should include a simple flag when calling `phi_model` that will exclude `id`, `created_at`, and `updated_at` from PHI wrapping. Instead of hard-coding ID, we should grab `class.primary_key` to know which one is the primary key.

Start at the `phi_model` entry point and trace how wrapped attributes are selected. Add an opt-in flag that excludes `id`, `created_at`, and `updated_at`, using `class.primary_key` rather than a hard-coded ID; done means these defaults are omitted when the flag is enabled.

Written by the indexing model from the issue text.