This change adds support for optionally auto-generating an IPv6 link-local
address based on the NIC's MAC Address on NIC enable.
Note, this change will not break existing uses of netstack as the default
configuration for the stack options is set in such a way that a link-local
address will not be auto-generated unless the stack is explicitly configured.
See `stack.Options` for more details. Specifically, see
`stack.Options.AutoGenIPv6LinkLocal`.
Tests: Tests to make sure that the IPb6 link-local address is only
auto-generated if the stack is specifically configured to do so. Also tests to
make sure that an auto-generated address goes through the DAD process.
PiperOrigin-RevId: 276059813
Ghanan Gowripalan