feat: config aliases (#18140)

Rename `no_memories_if_mcp_or_web_search` →
`disable_on_external_context` with backward compatibility

While doing so, we add a key alias system on our layer merging system.
What we try to avoid is a case where a company managed config use an old
name while the user has a new name in it's local config (which would
make the deserialization fail)

codex-rs/config/src/lib.rs

@@ -4,6 +4,7 @@ pub mod config_toml;
 mod constraint;
 mod diagnostics;
 mod fingerprint;
+mod key_aliases;
 mod marketplace_edit;
 mod mcp_edit;
 mod mcp_types;