Provide ability to specify a subnet range from your own subnet in management IP allocation

This idea was imported from Canny. Originally created by: Enkel Prifti. The current owner is: Unassigned.

Currently when using your own subnet for provisioning an instance, the API will allow you to select the elastic IP block but it does not provide the option to specify a range within that block. For example let say a customer has a /30 IP block containing 4 IPs. When the whole block is available, they would like to use the second /31 available in that block instead of the first. We should add a dropdown list that dynamically changes as the customer selects the subnet size, for example if they select /31 subnet size the drop down list will show both blocks available within the /30 elastic IP block that the customer can specify to use. The improvement should be similar to what we currently offer when attaching an Elastic IP to an instance where it shows all the available blocks within an Elastic IP subnet.