Add PoC exploit for CVE-2019-17558

[postmodern]

Add a PoC exploit for CVE-2019-17558.

Reference PoCs

• https://github.com/Ma1Dong/Solr_CVE-2019-17558/blob/master/Solr_CVE-2019-17558.py
• https://github.com/zhzyker/exphub/blob/master/solr/cve-2019-17558_cmd.py

Vuln App

• https://github.com/vulhub/vulhub/tree/639e32be6c0425b11d7d56594761c5e599a6a9d5/solr/CVE-2019-17558

How to Submit a PoC

See the CONTRIBUTING file for instructions on how to submit a PoC exploit.