lib/repo: Define a metadata key, ostree.deploy-collection-id

lib/repo: Define a metadata key, ostree.deploy-collection-id

This commit defines a metadata key that tells clients to update their
remote config to add a collection ID. This functionality is currently
implemented in Flatpak for the key "xa.collection-id", but there are two
good reasons for moving the key to OSTree:

1) Servers such as Flathub shouldn't set xa.collection-id in their
metadata now or in the medium term future, because many users are still
using old versions of Flatpak and OSTree[1] which would hit various
bugs[2][3][4] on the P2P code paths that are enabled by collection IDs.
Defining a new key means that only clients running recent
(as-yet-unreleased) versions of Flatpak and OSTree will pay attention to
it and deploy the collection ID, leaving the users on old versions
unaffected.

2) OSTree is as "invested" in collection IDs as Flatpak, so there's no
reason the key should be defined in Flatpak rather than here. According
to Philip Withnall, the reason the key was put in Flatpak originally was
that at the time there was uncertainty about tying OSTree to collection
IDs.

[1] https://ahayzen.com/direct/flathub.html#downloadsbyflatpakstacked
[2] https://github.com/ostreedev/ostree/commit/e4e6d85ea
[3] https://github.com/flatpak/flatpak/commit/5813639f
[4] https://github.com/flatpak/flatpak/commit/5b21a5b7

Closes: #1726
Approved by: pwithnall