Add a Cargo build profile for benchmarking (#21574)

A clean release build takes ~18m and an incremental build takes ~12m.
This is far too slow to iterate on performance related changes and the
build time is dominated by LTO.

This pull request adds a `profiling` profile for Cargo which takes ~13m
clean and ~6m incremental, the primary change is that LTO is disabled.
This matches a profile used in uv and follows the great work at
https://github.com/astral-sh/uv/pull/5955 — there's a bit of commentary
there about the trade-offs this implies.

We've found that this does not inhibit the ability to accurately
benchmark as measurements with LTO disabled are generally consistent
with the results with LTO enabled and it makes it much faster (~2x) to
rebuild after making a change.

This is motivated by my interest in improving Codex TUI performance,
which is blocked by the tragically builds right now.

I tested incremental build times by making a no-op change to the
`codex-cli` crate.

codex-rs/Cargo.toml

@@ -493,6 +493,12 @@ strip = "symbols"
 # See https://github.com/openai/codex/issues/1411 for details.
 codegen-units = 1
 
+[profile.profiling]
+inherits = "release"
+debug = "full"
+lto = false
+strip = false
+
 [profile.ci-test]
 # Reduce binary size to reduce disk pressure.
 debug = "limited"