Add extensions.yaml (subset of RHCOS)

[cgwalters]

This is a subset of the extensions.yaml from https://github.com/openshift/os

My immediate motivation is that day to day I develop in a Fedora toolbox targeting FCOS, and I wanted to test rpm-ostree compose extensions as cosa buildextend-extensions invokes it, and it's easier if we have a file here.

Since the FCOS pipeline doesn't invoke cosa buildextend-extensions, this will be a no-op.

Or stated more strongly: We might just use this as part of e.g. rpm-ostree's CI runs.

But...one could imagine that we actually share the list of extensions between FCOS and RHCOS in the future.

Or more obviously, perhaps we actually do something with this FCOS extensions list and don't actually offer "all Fedora packages" as extensions.

[jlebon]

How much of the developer pain is actually due to https://github.com/openshift/os/issues/552 and so not being able to just build RHCOS in the same toolbox on which you hack on rpm-ostree?

[bgilbert]

Hmm, if we don't (yet) have a concrete use for this, I'd say I'm weakly opposed.

[LorbusChris]

FWIW we do support extensions in OKD, so maybe it can be put into okd-machine-os for now?