[Fleet] Create .fleet-secrets system index

[hop-dev]

Part of elastic/kibana#154715

The .fleet-secrets system index should just have a value key.

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only.

Example document:

// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}