Remove dependency on rust_decimal, remove ignore of RUSTSEC-2026-0001 (apache#19666)

- Part of apache#19656

rust_decimal is a one person crate and is released somewhat infrequently
* https://github.com/paupino/rust-decimal
* https://crates.io/crates/rust_decimal

It also uses a non trivial number of dependencies, including
https://crates.io/crates/rkyv, some sort of zero copy deserialization
framework that was recently subject to a RUSTSEC security advisory, see
apache#19656 /
apache#19657

Since `rust_decimal` is only used for sqllogictests to parse the results
from postgres, we can pretty easily remove the dependency on
`rust_decimal` and inline the very small amount functionality we need
for sqllogictests

This will both decrease the build time and our dependency trail.

Removes the `rust_decimal` dependency from DataFusion
and inlines the smallest required subset of decimal functionality we
need for
sqllogictests (which turns out to be pretty small)

Yes by CI

No, this is all internal testing infrastructure

Author: alamb

.github/workflows/audit.yml

@@ -46,4 +46,6 @@ jobs:
         with:
           tool: cargo-audit
       - name: Run audit check
+        # Note: you can ignore specific RUSTSEC issues using the `--ignore` flag ,for example:
+        # run: cargo audit --ignore RUSTSEC-2026-0001
         run: cargo audit

Cargo.lock

@@ -55,9 +55,7 @@ indicatif = "0.18"
 itertools = { workspace = true }
 log = { workspace = true }
 object_store = { workspace = true }
-postgres-protocol = { version = "0.6.7", optional = true }
 postgres-types = { version = "0.2.11", features = ["derive", "with-chrono-0_4"], optional = true }
-rust_decimal = { version = "1.38.0", features = ["tokio-pg"] }
 # When updating the following dependency verify that sqlite test file regeneration works correctly
 # by running the regenerate_sqlite_files.sh script.
 sqllogictest = "0.28.4"
@@ -75,7 +73,6 @@ postgres = [
     "bytes",
     "chrono",
     "postgres-types",
-    "postgres-protocol",
     "testcontainers-modules",
     "tokio-postgres",
 ]