WIP: Virtual Network housekeeping: iotas -> enums; raw/polish -> marshal/unmarshal

[chrismarget-j]

WIP / todo:

• run integration tests against all supported apstra versions

This PR was supposed to be where I introduced individual control for VN -> leaf binding and SVI addresses.

But there was so much technical debt to clean up.

So, one step at a time.

In this PR we have:

• converted many old iota types to our generated enums (breaking change for SDK users)
• eliminated the raw and polish pattern in favor of implementing the json.Masharler and json.Unmarshaler interfaces.
• introduced some new helper functions:
  • func ipFromString(s string) (net.IP, error)
  • func ipNetFromString(s string) (*net.IPNet, error)
  • func macFromString(s string) (net.HardwareAddr, error)
• tightened up some test code with github.com/stretchr/testify/require
• introduced a unit test for virtual network marshal