Remove distutils from Burp plugin

[Mirochill]

Summary

• Replace the Burp plugin's distutils.util.strtobool call with a local boolean parser.
• Keep the accepted boolean string values compatible with the previous strtobool behavior.
• Add focused coverage for Burp request/response binary node decoding and invalid boolean values.

Fixes #35

Validation

• git diff --check HEAD~1..HEAD
• Not run locally: test suite, pre-commit