Gateway.Spec.Addresses.Value should be optional

[robscott]

What would you like to be added:
I'd like Gateway.Spec.Addresses.Value to be optional.

Why this is needed:
It should be valid to request a type of address without specifying the value that address should have. For example, on Google Cloud, we have a Cross Region ILB and we'd like users to be able to request ephemeral addresses in specific locations. That is currently quite challenging when the value of a requested address must be specified.

Note: Per our versioning policy it is valid to make required fields optional in a minor release across any release channel.

[EyalPazz]

/assign