examples: Add bootc UKI & BLS examples

[travier]

Related to:

• Composefs-native backend bootc-dev/bootc#1314
• composefs-rs integration in bootc bootc-dev/bootc#1190

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Those are the Containerfiles and scripts that we used for the DevConf.cz demo.

Draft for now until we clean things up and wire CI & bootc integration.

[cgwalters]

In chatting at devconf one thing came up I think would make sense:

• Make this crate only something like "composefs-storage"
• Things related to booting go into bootc-dev/bootc, but as a nicely factored out separate crate as is here so in theory other projects could use it or at least easily fork it

So things like this PR would basically go in bootc.

[travier]

So things like this PR would basically go in bootc.

Fine with me as well. I can move that there. Ideally we would have some CI here that checks those as well.