shibaken: return empty key list if IMDS returns 404

[jpculp]

Issue number:

N/A

Description of changes:

In the case where there are no keys available in IMDS, shibaken will create an empty public key list instead of returning an error.

This addresses a rare scenario where a user launches an instance of Bottlerocket without attaching a key to the EC2 instance. Rather than returning an empty string, the IMDS request for available keys returns a 404. The 404 status causes shibaken to return an error and prevents Bottlerocket from booting.

Testing done:

Set IMDS_PUBLIC_KEY_BASE_URI to an invalid URI and cargo ran shibaken

@webern
I have tested these changes. Before these changes an instance without a key failed to boot. With these changes the instance came up and I ran an ECS task.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[tjkirch]

Would you please also add a commit that adds this PR ID/link to the changelog entry for the shibaken set of changes?

[webern]

I have tested these changes. Before these changes an instance without a key failed to boot. With these changes the instance came up and I ran an ECS task.

[webern]

Upon most variant arches passing CI. :)

[jpculp]

• Added full link to the PR in the changelog.

[jpculp]

• Rebased and addressed merge conflict in CHANGELOG.md