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

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: https://github.com/krisprice/ipnet/pull/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.

No files or tests are named. Start by locating the current ipnetwork uses and Diesel Inet conversion boundaries, then review ipnet's JsonSchema support and the feasibility of its ToSql and FromSql converters. Done means the migration's scope and compatibility are established, or the switch is ruled out.