luci-base: return ipv6 lease time from getExpiry()

luci-base: return ipv6 lease time from getExpiry()
This commit introduces a streamlined method for retrieving lease time
 information on IPv6 interfaces.

While IPv4 interfaces typically have a single lease time (available
 in the interface data), IPv6 interfaces commonly feature multiple
 lease times due to factors like delegated prefixes or multiple /64 subnet.

Users typically receive only one prefix delegation from their ISP, leading
 this PR to the retrieval of lease time from the first available prefix
 or IPv6 address.

To enhance usability and provide users with a more comprehensive view
 of lease time information, however, alternative approaches
 should be discussed.

Suggestions include returning an array of lease times or aggregating
 the various possible values using methods like min or max.

These enhancements aim to improve the clarity the lease time
 management on IPv6 interfaces.

Signed-off-by: hitech95 <nicveronese@gmail.com>