Add new targeted script add_junk

[5ubterranean]

Targeted script that works similar to this plugin https://github.com/assetnote/nowafpls. it's purpose is to add junk data to a request to try to bypass WAFs.

[kingthorin]

To address the DCO requirement you'll need to sign-off the commit(s):

• https://github.com/zaproxy/zaproxy/blob/main/CONTRIBUTING.md#developer-certificate-of-origin
• https://git-scm.com/docs/git-commit#Documentation/git-commit.txt---signoff

[kingthorin]

The CHANGELOG.md should also be updated.