Follow up to elastic/kibana#180778

After the work in #3090, the log level for an agent can now be set (and unset) at various levels.

Currently agent simply reports a local_metadata.elastic.agent.log_level which does not indicate where it retrieved the log level from.

Suggest to expose another field which tells the user and the UI where the log level is sourced from, i.e.:

local_metadata.elastic.agent:
      log_level_source: 'agent_setting' | 'agent_policy' | 'default'

Once this is added, please open a followup Kibana issue so that Fleet can read it to conditionally display Apply changes and Reset to policy on the agent log level selector UI. Today we have to always show both actions because we do not have this information: