[Fleet] Add `data_backfill` migration for `supports_agentless` field on agent policies + package policies

[kpollich]

In #199567, we added a supports_agentless property to package policies, but we did not properly backfill this data for existing policies in use by agentless customers. We need to add a new SO migration to backfill this property on existing policies that run agentless integrations in order for those existing policies to show up in the agentless UI.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

I think the backfill should be done by setting supports_agentless on package policies where the agent policy has supports_agentless. I think the agent policies already had this flag populated for existing projects.

However we can't do this in SO migrations, because we only have access to package policy fields, and can't query agent policy SO from there.
We could do the backfill by adding a function to Fleet setup that queries agent policies and updates package policies. Are we ok with that?