Add configurable captive portal probes

The probes allow testing for a configurable status code and location
header (regexes). They are disabled by default, so this CL is a
no-op unless the probe configurations are pushed.

Bug: b/79499239
Test: tests in CL pass, manual: captive portal login works
Change-Id: I785723aaed06054b9aa8ebff77803f23d7836db9