Replace From<Ip*Addr> for IpNet with explicit constructors

[jgallagher]

After running into a mistake where we converted a raw IP into a /128 (when it should have been a /56), fixed in #5668, we filed an upstream request (achanda/ipnetwork#190) to remove the From impl to discourage accidental misuse by requiring a more explicit constructor. I just noticed we also have an IpNet type that implements these same conversions:

omicron/common/src/api/external/mod.rs

Lines 1451 to 1455 in 50cd41c

	impl From<Ipv4Addr> for IpNet {
	fn from(n: Ipv4Addr) -> IpNet {
	IpNet::V4(Ipv4Net(ipnetwork::Ipv4Network::from(n)))
	}
	}

(and the subsequent conversions from Ipv6Addr and IpAddr a little below that). We should also remove these and provide explicit constructors instead, I think.