all: implement RFC8910 captive portal (CP) option

all: implement RFC8910 captive portal (CP) option

https://www.rfc-editor.org/rfc/rfc8910.html

RFC8910 defines a captive portal API URI for a CP client to consume.

With captive_portal_uri set to 'https://test.example.com'

Produces in RA:

ICMPv6 Option (DHCP Captive-Portal)
    Type: DHCP Captive-Portal (37)
    Length: 4 (32 bytes)
    Captive Portal: https://test.example.com

And in DHCPv6 Reply:

Captive Portal
    Option: Captive Portal (103)
    Length: 24
    Captive Portal: https://test.example.com

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/315
Signed-off-by: Álvaro Fernández Rojas <[email protected]>