[#64583056] Recommit load balancer

This PR re-adds the reverted changes from  https://github.com/alphagov/vcloud-tools/pull/159 (on vcloud-tools, before splitting into vcloud-edge_gateway)

The primary fix needed was a missing 'require: false' in the load_balancer schema, but there are some minor tweaks to reshape for the split into the independent Gem.

The original PR, and hence these commits, added:

* LoadBalancerService generator and schema checking code to the main update functionality.
* new Differ class, that handled removing a dynamic field from the remote configuration before comparison.