Add support for `PATCH`

At the moment only `POST` and `PUT` are supported.
Adding `PATCH` makes sense, but we'll be using the standard JSON serializer. This way we don't have to assume things what we have to do with partial updates, `null` values, etc.

This is up to the API or creator of the DTO.

Start by locating where POST and PUT are handled and how the standard JSON serializer is selected. Confirm the expected behavior with the repository’s existing checks, if any; done means PATCH is accepted without adding custom partial-update or null-value assumptions.

Written by the indexing model from the issue text.