dhcpcd: Request and report Web Proxy Auto-Discovery

Request the Web Proxy Auto-Discovery DHCP option, and report
any retrieved value over over DBus.

BUG=chromium:253915
TEST=Packet capture from network_DhcpNegotiationSuccess autotest
Turn on dhcp debugging in shill and note that a WebProxyAutoDiscoveryUrl
key is received (but ignored).

Change-Id: Ib3a0fe4fd4559c7f61545e2a64077265adf0fe7c
Reviewed-on: https://gerrit.chromium.org/gerrit/59967
Reviewed-by: mukesh agrawal <[email protected]>
Tested-by: Paul Stewart <[email protected]>
Commit-Queue: Paul Stewart <[email protected]>
3 files changed