feat: introduce codex_execpolicy crate for defining "safe" commands (#634)

As described in detail in `codex-rs/execpolicy/README.md` introduced in
this PR, `execpolicy` is a tool that lets you define a set of _patterns_
used to match [`execv(3)`](https://linux.die.net/man/3/execv)
invocations. When a pattern is matched, `execpolicy` returns the parsed
version in a structured form that is amenable to static analysis.

The primary use case is to define patterns match commands that should be
auto-approved by a tool such as Codex. This supports a richer pattern
matching mechanism that the sort of prefix-matching we have done to
date, e.g.:


https://github.com/openai/codex/blob/5e40d9d2211737f46136610497bcd9a8271009e0/codex-cli/src/approvals.ts#L333-L354

Note we are still playing with the API and the `system_path` option in
particular still needs some work.

codex-rs/Cargo.toml

@@ -6,6 +6,7 @@ members = [
     "cli",
     "core",
     "exec",
+    "execpolicy",
     "interactive",
     "repl",
     "tui",