TUF: Add root status output

[asraa]

Signed-off-by: Asra Ali [email protected]

Summary

Add structured output describing the TUF root, for example:

$ rm -r $HOME/.sigstore && ./cosign initialize
Root status: 
 {
	"local": "/home/asraa/.sigstore/root",
	"remote": "sigstore-tuf-root",
	"expiration": {
		"root.json": "11 May 22 19:09 UTC",
		"snapshot.json": "15 Feb 22 00:39 UTC",
		"targets.json": "11 May 22 19:10 UTC",
		"timestamp.json": "15 Feb 22 00:39 UTC"
	},
	"targets": [
		"rekor.pub",
		"artifact.pub",
		"ctfe.pub",
		"fulcio.crt.pem",
		"fulcio_v1.crt.pem"
	]
}

Ticket Link

Fixes #1297

Release Note

* Adds TUF root status with expiration and repository locations to output of `cosign initialize`