Add API for CaptivePortalData

CaptivePortalData will be used to hold data advertised by the network
following RFC7710bis.

To fetch the CaptivePortalData, the API URL is added to LinkProperties,
to be provided by the NetworkAgent.

Because CaptivePortalData can be used to guess user location (especially
from the URLs provided by the portal), it is only exposed to
applications that have privileged permissions.

Test: atest FrameworksNetTests
Bug: 139269711
Change-Id: I341175b5fece8ee00e19898af5e8eabe66cefbf3