Close specific engine instance

[yaron12n]

Proposed changes

Add API for specific engine close.
5821

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[GeorginaReeder]

Thanks for your contribution @yaron12n ! :)

[tarunKoyalwar]

@yaron12n , pls refer to docs related to library https://pkg.go.dev/github.com/projectdiscovery/nuclei/v3@v3.3.5/lib#section-readme

due to current design (global state variables) . running two instances of NewNucleiEnginexxx gives unexpected results because it overwrites internal global state to avoid this we have added a ThreadSafeNucleiEngine which handles this and reuses the same internal global state and allows running multiple instances in parallel via ExecuteNucleiWithOpts see example in sdk documentation

in this pr you are correctly closing instance only variables but if you check init method the global state is set everytime you run so that issue still remains

we have planned to refactor and remove global state but no ETA on that

[tarunKoyalwar]

i am currently closing this issue because it doesn't resolve the global state issue , feel free to reopen with more context if you think otherwise