less allocations in parse_svcb_ip

[jrouaix]

This small PR change the parse_svcb_ip function impl.
It's a little more complicated (sorry for that), but reduce the number of allocations :

n4 : number of ipv4 addresses
n6 : number of ipv6 addresses

Before : 2 + n4 + n6 + 2 + 1 (2 vecs + to_string for IPs + 2 joins + format!)
After (best case) : 2 + 1 (2 string buffers + format! in the end)

In the worst case, if there are many many ip addresses, String growth will reallocate the buffers, so it might be not that interesting.

What is the expected/usual number of ip addresses here ?

[puffyCid]

PR looks fine, thanks!

What is the expected/usual number of ip addresses here ?

i do not know the typical number.