[Elastic Agent] Implement dynamic reloading of the agent policy

As part of the [work](https://github.com/elastic/kibana/issues/133828) for configuring custom `source_uri` in Fleet UI, we need to be able to configure  `agent.download.sourceURI` in the Agent Policy, but at the moment the agent is not able to reload the policy at runtime. Could you help out with this feature?

The other option would be to pass the custom `source_uri` through the fleet-actions, but this would impact the scalability in the case of bulk upgrades (https://github.com/elastic/kibana/issues/135579).