Update sign output to note when signatures are pushed

[haydentherapper]

This updates the output for cosign sign to match what's in the documentation. A few other output cleanups:

• Changed when the newline is written for fetching a password
• Removed writing the signature to stdout, only writing the signature to a file when requested

New output:

$ ~/go/bin/cosign sign --key cosign.key us-west1-docker.pkg.dev/project/docker-repo/image:tag1
Enter password for private key:
Pushing signature to: us-west1-docker.pkg.dev/project/docker-repo/image

Previous output:

$ ~/go/bin/cosign sign --key cosign.key us-west1-docker.pkg.dev/project/docker-repo/image:tag1
Enter password for private key: MEYCI....

Release Note

NONE

[priyawadhwa]

Lgtm!