Add Brekeke SIP Server Zip Slip real-world case

[Exploit-Garbage]

What does this PR add?

A real-world Zip Slip exploitation case, added to Zip Slip/README.md
under a new ### Real-world case subsection in ## Methodology, plus a
reference link in ## References.

Case: Brekeke SIP Server — Unauthenticated Zip Slip Webshell RCE

A self-developed Zip.extractAll method in the ProvisioningModelImport
bean of Brekeke SIP Server (v3.19.1.8p1) performs no .. filtering or
canonical-path validation. A single unauthenticated POST uploading a
crafted model archive writes a JSP webshell into the Tomcat webroot, and
a single GET triggers it — RCE as the tomcat user, no credentials,
factory default configuration (CVSS 9.8).

The contribution includes:

• A concrete payload: building a malicious zip with a 7-layer ../
  traversal entry pointing at the webroot.
• The unauthenticated upload request (ProvisioningModelImport bean via
  GateServlet) and the webshell trigger request.
• A reference link to the full advisory with root-cause analysis and a
  self-contained exploit script.

This matches the existing section style (methodology + concrete
payloads + references) and adds a documented, reproducible real-world
instance of the vulnerability class.

Checklist

• Content placed in the correct section (Zip Slip/README.md)
• Follows the existing format (methodology + code blocks + references)
• Payloads use placeholder targets (<target_base_url>, 127.0.0.1)
• No promotional language
• Reference link resolves over HTTPS