Save retained properties in ecm:retainedProperties

With the introduction of NXP-31396, we'll need a way to let clients and 3rd parties know that some properties of a document are not editable because under retention.

At the time a retention rule is attached to a document, it becomes a record and the record facet along with the record schema (https://github.com/nuxeo/nuxeo-retention/blob/lts-2021/nuxeo-retention/src/main/resources/schemas/record.xsd) is added to the document. We'll store as a property of this schema the list of the metadata to be retained defined from the core schema contribution allowed by NXP-31396.

This list will be available under

document.properties['record:retained properties']

EDIT

We have an issue with the evolution NXP-31396 merged in https://github.com/nuxeo/nuxeo-lts/commit/7ed49bb2d29bd945a7c1d918b13963b8f349db2a#diff-da848c34bfe8aa263fb1e2ec526af1d10dc1f187e8125e92cca23a2c3ff01aacR895

Considering the retainable property extension can evolve over time, let's consider the following flow:

1. a contribution makes files:files/*/file a retainable property
2. document A with attachments (files:files content) is put under retention
3. the initial contribution evolves and files:files/*/file is no longer a retainable
4. a user tries to nullify files:files of document A
5. it is possible because we check the current contribution, not the retained properties at the time doc A was put under retention
6. KO

We should store the retained properties of a doc at the time it is put under retention in a system property to make sure retained properties remains retained whatever the current contribution.