Offline signing flag

[aaronlew02]

Description

v4 will require that users who need services for signing must specify those services via a signing config. As of #4959, this can be done either by providing a signing config explicitly (via --signing-config) or relying on one fetched automatically by TUF (--use-signing-config). Users signing offline must set --use-signing-config=false.

Since --use-signing-config=false was originally intended to allow for specification of services via URL flags instead of a signing config, the flag can become semantically confusing for users.

This issue proposes removing --use-signing-config in favor of:

• Signers requiring TUF services simply omitting --signing-config
• Signers NOT requiring TUF services omitting --signing-config and using a new flag (e.g., --offline)