Add WSTG-INPV-22: Testing for Insecure Deserialization (Fixes #7)

[Galaxy-sc]

This PR covers issue #7.

• This PR handles the issue and requires no additional PRs.
• You have validated the need for this change.

What did this PR accomplish?

Added a comprehensive guide for Testing for Insecure Deserialization (WSTG-INPV-22). Key highlights:

• Identification: Patterns for Java, PHP, Python, .NET, and Node.js.
• Modern Vectors: Added JSON deserialization (Jackson/Fastjson) and Prototype Pollution.
• Exploitation: Structured strategy for Gadget Chains and OOB testing.
• Remediation: Focus on integrity checks and digital signatures.

Fixes #7

[kingthorin]

Please be aware that I won't be super active this week or next. If you aren't getting reviews it isn't for lack of appreciation, I'm just busy with some family stuff.