HaikuDepot: Url -> Identifier

This change will rename the confusing "url" within
HaikuDepot to be "identifier" in line with
corresponding changes in pkg kit and HDS. Also at
the same time support is introduced for HDS
repos' meta-data to artificially match against
multiple repos; as requested for the future R1B3
release process. Some tidy-ups and extensions have
been made to the JSON schema-to-model and the
schema-to-parser scripts.

Change-Id: I402e7d610986039f58d72028bda7de977e9115e2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2986
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

HaikuBook: Add documentation for Password and Key Storage API

The current implementation of the keystore_server is not perfect. While the
source has many seeds for a future of having keyrings encrypted, and having
more fine grained permissions, it is far from complete.

The main arguments for adding documentation about this new but incomplete
functionality is that while it is incomplete, the API is part of the public
headers, and there are some legitimate use cases for developers.

The documentation aims to give the proper amount of caution to any developer
that is considering using this API.

Change-Id: I154a3f8374b22dc6929758cba7ba810833bcfe9d

API Docs: Add placeholder for Key.h, KeyStore.h and Notification.h