Implement parsing of the <id> element in OPDS 1.x feeds

[x0000ff]

The OPDS 1.x is based on Atom, and Atom’s Feed must contain an id tag. I found it useful to parse the <id> tag.

1.1. Introduction
The Open Publication Distribution System (OPDS) Catalog format is a syndication format for electronic publications based on Atom [RFC4287] and HTTP [RFC2616].

atom:feed elements MUST contain exactly one atom:id element.

I left it as optional to avoid breaking compatibility with existing clients.

• ✅ Tests are updated

[mickael-menu]

Looking good, thank you. Please note that we will revamp the OPDS models next year, and this kind of information will be available as part of the extension models for metadata instead of a native property.