feat: add ability to export features for static node

[vsoch]

Problem: We want to statically export NFD features for HPC. Solution: Add an nfd export features mode that allows for generation and export of static feature labels.

For the HPC use case, we want to be able to export static features, either to the screen (for inspection) or a file (for saving and use with a scheduler, or other compatibility checking algorithm). Adding support for this is fairly easy - we need to just add the subcommand. The usage is simple and intuitive:

# Export labels to the terminal for inspection
nfd export features

# Save to a file path
nfd export features --path features.json

The flag is also useful for cases beyond HPC when a quick inspection is desired. There are ways this could be extended (requesting specific groups or filters) but I think it's better to start simple.

This is a follow up of https://github.com/kubernetes-sigs/node-feature-discovery/pull/2170. If you'd like tests added, I did an attempt and it looks like the features aren't exposed in the test environment - we would need a mock. Please point me to an example for how to do that. Thanks!

[k8s-ci-robot]

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[mfranczy]

That's a nice functionality, not only for non-k8s use cases, but also for taking a snapshot of the node's state from the perspective of enabled features.

[ChaoyiHuang]

/lgtm

[vsoch]

Ping @adrianchiris @zvonkok @ArangoGutierrez @marquiz we've gone through several rounds of review and have LGTM from the compatibility working group. Can any of you review this?

[ArangoGutierrez]

/ok-to-test

[fmuyassarov]

/test pull-node-feature-discovery-build-image-cross-generic

[fmuyassarov]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3550e71026ecb04e82ea603e382e519aa2204a5e

[ArangoGutierrez]

/test pull-node-feature-discovery-verify-master

[ArangoGutierrez]

@vsoch please check the failing test, PR is now approved

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: ab5182d1cfd0b36399167cdd4c2d287d278fe909

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ArangoGutierrez, mfranczy, vsoch

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[vsoch]

Thanks to all! :tada: