Add captive portal info to DhcpClient output

Requesting the captive portal option is flagged off by default.
The URL it provides will be used to support the captive portal API; see
RFC7710bis.

Bug: 139269711
Test: atest NetworkStackTests NetworkStackNextTests
Test: atest NetworkStackIntegrationTests NetworkStackNextIntegrationTests

Change-Id: I783466e0e60f364e79cd76af3fe43a7862d35cf2