feat: add --existing-version flag to attestation init (#2541) #308

Job	Run time
test / Test (cli)	1m 55s
test / Test (main-module)	3m 50s
test / Test (artifact-cas)	39s
test / Test (controlplane)	6m 20s
init_attestation	12s
Release CLI and control-plane/artifact-cas container images	12m 28s
Finish Attestation	8s
modify_release_notes	6s
	25m 38s

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