Add source_ipv6_prefixes parametr to nat64stateful

[glumpo]

Adds optional source_ipv6_prefixes field support to nat64stateful modules.

The control plane now:

• Selects a NAT64 module using both source and destination IPv6 prefixes.
• Preserves existing behavior with ::/0 as the default source prefix.
• Exposes source prefixes in the nat64stateful CLI output.

[ezhk]

There is a source-prefix parsing issue to fix before merge.

source_ipv6_prefixes validates the mask with std::stoul(mask_string) (decimal), but then constructs ipv6_prefix_t from the original string. That constructor parses the mask with base 0, so leading-zero masks are interpreted as octal: for example, 2001:db8::/064 passes validation but becomes /52, while ::/08 becomes /0. This can make the generated ACL select the NAT64 module for a much broader source range than configured.

Please construct the prefix using the already-parsed decimal mask (or reject noncanonical leading-zero masks).

The added autotest currently checks only the default ::/0 CLI output. It would also be valuable to configure a non-default source prefix and verify packets from both matching and non-matching source ranges.