chore(l2): insert placeholder proxies for system contracts

[ManuelBilbao]

Motivation

Inserting placeholders allow us to later include new system contracts in already deployed networks by just upgrading the proxies

Description

[Copilot]

Pull Request Overview

This PR adds placeholder proxies for system contracts in the L2 genesis configuration, enabling future upgrades to include new system contracts in already deployed networks by upgrading these pre-deployed proxies.

• Introduces add_placeholder_proxy function to create proxy contracts without implementation
• Deploys placeholder proxies for address range 0xff00 to 0xfffc (253 proxies)

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[github-actions]

Lines of code report

Total lines added: 23
Total lines removed: 0
Total lines changed: 23

Detailed view

+-------------------------------+-------+------+
| File                          | Lines | Diff |
+-------------------------------+-------+------+
| ethrex/cmd/ethrex/build_l2.rs | 397   | +23  |
+-------------------------------+-------+------+