feat: Add allowing retrieving all fields for an item

[capripot]

With 1Password, there is a way to retrieve all fields of a given item directly without having to enumerate them.

Allowing this when passing no arguments for secrets fetch command.

For example:

kamal secrets fetch --adapter 1password --account ABCD1234 --from Vault/myitem

would return the expected same JSON.

That can be particularly useful when you are not using rails credentials and have a lot of keys set in .kamal/secrets.

That would also allow the current example to shorten:

SECRETS=$(kamal secrets fetch --adapter 1password --account my-account --from MyVault/MyItem)
KAMAL_REGISTRY_PASSWORD=$(kamal secrets extract KAMAL_REGISTRY_PASSWORD $SECRETS)
RAILS_MASTER_KEY=$(kamal secrets extract RAILS_MASTER_KEY $SECRETS)