[dependencies] Consider switching from ipnetwork to ipnet for IP network management

[smklein]

ipnetwork - currently used in omicron - and ipnet (not used yet) serve very similar purposes: they both track network addresses, ranges, and subnets.

Background

• We originally began using ipnetwork as it was supported natively within Diesel (see: the conversion for https://docs.diesel.rs/master/diesel/pg/sql_types/struct.Inet.html ). ipnet didn't have such converters.
• Additionally, neither type implemented JsonSchema - we had to hand-roll that ourselves.

Progress

• The ipnet crate has been pretty amenable to adding JsonSchema support: Add 'schemars' feature for deriving JSON schema krisprice/ipnet#31
• It seems relatively straightforward (though I haven't tried) to add ipnet converters ToSql and FromSql for Diesel

Doing so would let us move to a crate with slightly better subnet-management functionality, and a better cadence of support.

[smklein]

Adding ipnet support to diesel has been proposed with diesel-rs/diesel#3164 , and implemented with diesel-rs/diesel#3165

[ahl]

Maybe relevant: achanda/ipnetwork#157

[bnaecker]

Also: diesel-rs/diesel#3244

It seems that this now means we can just use ipnet and its types directly, both in the external API and the database representation.