Added a11y metadata for exemptions

[HadrienGardeur]

The European Accessibility Act provides a number of exemptions that apply to publications that do not conform to its requirements.

These exemptions could already be expressed in ONIX but they were missing in EPUB. This has recently changed with the publication of a W3C Group Note.

This PR is based on this note:

• it introduces a new exemption metadata with three values: eaa-disproportionate-burden, eaa-fundamental-alteration and eaa-microenterprise
• and adds them to our JSON Schema as well

This will be useful when parsing new EPUB files that may contain this metadata and exposing them as an RWPM, but also in OPDS 2.0 where in theory, all publications that do not meet requirements in the EU should contain this metadata.

[gautierchomel]

Good addition.

Actual discussions recommend never exposing that information, as it would be understandable and trick the user into thinking that the publication is not accessible to him (which is usually not true).

This information may be useful for filtering and should be stored by booksellers and libraries in case a user starts complaining so they can orient him to the right service.

See https://w3c.github.io/publ-a11y/UX-Guide-Metadata/2.0/principles/#legal-considerations

Publishers may need to include information about an exemption or exception in metadata for legal or clarity reasons, either to bodies that enforce legislation or for other business to business communication. However this is not information that needs to be displayed on public sites as it does not mean anything to most consumers and could lead to misunderstandings.

[HadrienGardeur]

This information may be useful for filtering and should be stored by booksellers and libraries in case a user starts complaining so they can orient him to the right service.

For a number of countries (for example in Spain, Finland, Canada and in the US), OPDS is used to transmit metadata to libraries. With this addition, they will be able to receive this information but it will be up to them to decide how to use it.

[HadrienGardeur]

I'm also pinging the maintainers as they'll need to implement support for this in shared models and when parsing EPUB files or OPDS cc @mickael-menu @qnga @chocolatkey @danielweck