odhcp6c: add new option to start 4in6 protos as dynamic interface

odhcp6c: add new option to start 4in6 protos as dynamic interface

If proto 'dhcpv6' is set for an interface, dynamic interfaces are created
for the protocols map, dslite or 464xlat if this netifd protocols are
installed and the interface option is not explicitly set to '0'.

The problem is that this option cannot be configured via LuCI, which means
that the dynamic protocols are started. In my case, that is the '464xlat'
'6in4' protocol. I see the follwing log messages continuously in the log as
I do not have a '464xlat' in my network.

Fri Aug 22 10:36:33 2025 daemon.notice netifd: Interface 'wan6_4' is now down
Fri Aug 22 10:36:33 2025 daemon.notice netifd: Interface 'wan6_4' is setting up now
Fri Aug 22 10:36:36 2025 daemon.notice netifd: Interface 'wan6_4' is now down
Fri Aug 22 10:36:36 2025 daemon.notice netifd: Interface 'wan6_4' is setting up now

To fix this by adding a new option to disable the dynamic interface creation
for '4in6' if needed. The option is named '4in6_dynamic' and is a boolean.

If the new option is 'true' (default) dynamic interfaces are create.
If the new option set to 'false' no dynamic 4in6 interface are created.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>