Add ExecPolicyManager (#8349)

Move exec policy management into services to keep turn context
immutable.

codex-rs/Cargo.lock

@@ -365,6 +365,12 @@ dependencies = [
  "x11rb",
 ]
 
+[[package]]
+name = "arc-swap"
+version = "1.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "69f7f8c3906b62b754cd5326047894316021dcfe5a194c8ea52bdd94934a3457"
+
 [[package]]
 name = "arrayvec"
 version = "0.7.6"
@@ -1256,6 +1262,7 @@ name = "codex-core"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "arc-swap",
  "assert_cmd",
  "assert_matches",
  "async-channel",