extensible metadata on deployments

[cgwalters]

As prep for #2725 but also in general, I think it'd be useful to have an official "metadata" API for deployments - surfacing things like "did this deployment boot before" in a way indepdenent of the journal.

I think this could be a pretty thin wrapper for extended attributes on the deployment root directory. (Need to verify whether the immutable bit breaks this; if it does we could just document using the origin file for this)

[cgwalters]

BTW if we do this we should also teach the OpenShift MCO to do this, to describe which rendered configs it used for the deployment.

[cgwalters]

One thought related to this is that with composefs, we automatically gain a new .cfs file that would a natural target for xattrs.

[cgwalters]

(And maybe we should just turn on composefs generation unconditionally by default?)

[jmarrero]

we discussed turning composefs for Fedora ELN first.

[cgwalters]

One thing that happened with #3114 is we gained a whole new directory created unconditionally now - so we can trivially drop in a new file there in whatever format we want (but probably gvariant) or xattrs on that directory.