feat: support for KMS keys in cosign signature

[brennoo]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

/kind flaky-test

Any specific area of the project related to this PR?

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/area library

/area cli

/area tests

/area examples

What this PR does / why we need it:
This PR adds support for KMS providers in the cosign signature verify process, making it possible to use cloud providers KMS, hashicorp vault and kubernetes.

Which issue(s) this PR fixes:

Fixes #380

Special notes for your reviewer:
I've tested key parameter with different values including http://, awskms:// and local file pubkey.pem - it was needed to add the private-infrastructure flag for cases where the transaction is not sent to the transparency log

[LucaGuerra]

cc @cpanato , as mentioned in the linked issue ;) would be very appreciated if you could look into the PR, you're the expert here!

[cpanato]

A few nits and also please squash the commits

[cpanato]

can you please update to v1.8.1 to match the dependency in line?

[cpanato]

I think this does not need if and elses can be just the entire thing as was before, because in the case before we can only have either CertificateIdentity or CertificateIdentityRegexp

so then it will be

v := cosign.VerifyCommand{
		CertVerifyOptions: options.CertVerifyOptions{
			CertIdentity:         signature.Cosign.CertificateIdentity,
			CertIdentityRegexp:   signature.Cosign.CertificateIdentityRegexp,
			CertOidcIssuer:       signature.Cosign.CertificateOidcIssuer,
			CertOidcIssuerRegexp: signature.Cosign.CertificateOidcIssuerRegexp,
                         // used when using the kms verify
			KeyRef:     signature.Cosign.KeyRef,
			IgnoreTlog: signature.Cosign.PrivateInfrastructure,
		},
}

I don't have a KMS to test, but is it possible for you to check for both cases? I think this will work, and it simplifies a lot.

Thanks for this PR

[poiana]

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