feat: rebrand trusted signing to artifact signing

[Jaxelr]

Resolves #971

This PR performs the following changes:

• Creates a new artifact signing command along with its corresponding options and bootstraps it as a new subcommand of the sign command.
• Shows a new message of obsoletion for the current trusted signing command.
• Renames the existing trusted signing provider to artifact signing while using 1 provider for both the existing and new command.
• Renames the corresponding unit tests and adds test for obsolete command
• Updates and renames existing trusted signing integration doc to artifact signing integration.