Minimize non-"meta" contents of pgrx::datum: uuid, json, net, num

[ChronicallyJD]

Summary

Continues #2194 by moving the remaining basic-datatype items out of pgrx::datum into dedicated top-level modules, following the datetime precedent from #2199. This covers the outstanding checklist items:

• uuid → pgrx::uuid
• json → pgrx::json
• net (inet) → pgrx::net
• numeric → pgrx::num (with its support code at pgrx::num::support)

Each type group is a separate commit.

Everything remains accessible from the previous paths via re-export, so this is purely organizational with no end-user impact except new path options. In particular, the module paths pgrx::datum::numeric and pgrx::datum::numeric_support are preserved, along with the re-exports flowing through pgrx::prelude and the crate root (pgrx::Uuid, pgrx::Inet, pgrx::Json, pgrx::JsonB, pgrx::AnyNumeric, pgrx::Numeric, pgrx::numeric).

pgrx::num uses the same num.rs + num/ layout that datetime.rs + datetime/ uses.

Note on clippy

Moving code out of datum drops the module-level #![allow(...)] on datum/mod.rs, so the numeric move surfaces two clippy::unnecessary_fallible_conversions warnings (tracked in #1525). This matches the datetime migration exactly: datetime.rs carries no such allows and already surfaces the same lint on develop. No new allows were added, to stay consistent with that precedent.

Testing

Against PostgreSQL 18:

• cargo test --all --features "pg18 pg_test cshim proptest" --no-default-features — all suites pass (977 tests, 0 failures, including the 603-test pgrx-tests integration suite).
• cargo fmt --all --check — clean.
• cargo clippy -p pgrx-sql-entity-graph -- -Dwarnings — clean.

Part of #2194.

[ChronicallyJD]

Note: the failing `verify package can build` check on this PR is a pre-existing issue on `develop`, unrelated to this change. That workflow builds with nightly, which promotes the `semicolon_in_expressions_from_macros` lint (rust-lang/rust#79813) to a hard error, tripping some `bail!(...)` calls used in expression position. This PR does not touch that code. Fixed separately in #2360.