Move environment abstraction into exec server (#15125)

The idea is that codex-exec exposes an Environment struct with services
on it. Each of those is a trait.

Depending on construction parameters passed to Environment they are
either backed by local or remote server but core doesn't see these
differences.

codex-rs/exec-server/src/environment.rs

@@ -0,0 +1,11 @@
+use crate::fs;
+use crate::fs::ExecutorFileSystem;
+
+#[derive(Clone, Debug, Default)]
+pub struct Environment;
+
+impl Environment {
+    pub fn get_filesystem(&self) -> impl ExecutorFileSystem + use<> {
+        fs::LocalFileSystem
+    }
+}