Allow runtime parsing of datasources using a config option

[jimmykarily]

There cases in which we want to keep pulling datasources at runtime (not just installation), for instance data centers.

Proposed alternative

• Let's make it a config option, which can also be passed through the cmdline, so people can enabled it back if they need it.
• Let's imply that pulling from datasources is allowed if not in Trusted Boot mode (preferred, no config required)

Additional context

We removed the functionality as part of this: kairos-io/packages#1137