attestation: create issuer based on kernel cmd line

[daniel-weisse]

Proposed change(s)

• Instead of relying on just the Cloud Provider to identify what Attestation Issuer we need to set up, we now use information from the Kernel command line to do so
• Add choose.Issuer() and choose.Validator() utility functions to create Issuer or Validator from a given oid

Additional info

• Set up of Issuer for the VerifyService will be handled in a different PR
• Dynamic creation of Validators using choose.Validator() will be handled in a different PR

Checklist

• Update docs
• Add labels (e.g., for changelog category)
• Link to Milestone

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[malt3]

Change so far looks good.
Do we also want to implement a chooser to select the validator for the joinservice? We would need to pass this as either an env var or a command line flag and then refactor internal/watcher/validator.go a bit.
What do you think? A TODO would be good enough for now.

[daniel-weisse]

Do we also want to implement a chooser to select the validator for the joinservice?

See my PR description. Related Ticket: AB#2975