Filled in IPv6Range object

Closes linode/linode-cli#207

It was reported above that the CLI wasn't showing complete output for
IPv6 Ranges compared to IPv6 Pools.  Turns out this is because the docs
were missing several fields for ranges.

This PR adds the missing fields to IPv6Ranges.  I based the descriptions
and values off of the equivalent fields in IPv6Pool.

Author: Dorthu

openapi.yaml

@@ -15300,13 +15300,26 @@ components:
           example: '2600:3c01::02:5000::'
           readOnly: true
           x-linode-cli-display: 1
+        prefix:
+          type: integer
+          description: >
+            The prefix length of the address, denoting how many addresses can be
+            assigned from this range calculated as 2 <sup>128-prefix</sup>.
+          example: 64
+          x-linode-cli-display: 2
         region:
           type: string
           description: >
             The region for this range of IPv6 addresses.
           example: us-east
           readOnly: true
-          x-linode-cli-display: 2
+          x-linode-cli-display: 3
+        route_target:
+          type: string
+          description: >
+            The last address in this block of IPv6 addresses.
+          example: '2600:3c01::02:5000::ca72'
+          nullable: true
     Kernel:
       type: object
       description: Linux kernel object