Add profile option for fleet server host URL in fleet server settings

Alike to the outputs profiles for Elasticsearch/Logstash and the Agent Binary Download that you now can specify in Agent policy settings, it would be very useful to also be able to do that for the Fleet server hosts part. This way if you have multiple fleet servers in separated networks or a fleet server that resides in DMZ vs a fleet server that services the internal network, you would be able to direct elastic agent to the right fleet server, the same as you now already can do that for the 2 other options.