Return SCT when a cert is generated

[priyawadhwa]

We have this TODO to actually return the SCT when a cert is created:

fulcio/pkg/api/signing_cert.go

Line 93 in 1046134

//TODO: return SCT and SCT URL

If we started doing this, we could use the SCT to verify the cert is in the CT log. This would be the final piece of validation for cosign bundling to be finished!

[priyawadhwa]

Not entirely sure if it would be OK to just change the API and return the SCT, right now we just return a string:

fulcio/openapi.yaml

Lines 48 to 51 in 39dcd1a

	description: Generated Certificate Chain
	schema:
	type: string
	400:

We could return the SCT as a JSON-encoded string and not have to change the API spec, but I don't love that option.

[bobcallaway]

as soon as we have sigstore/sigstore#70 or sigstore/sigstore#69 resolved (which should be pretty soon) I'll push up a patch to include the SCT in the final certificate we issue.