SDK: abstracted and minimal nuclei v3 sdk

[tarunKoyalwar]

Proposed changes

• New easy to use and simplified Library/SDK for nuclei
• Added testable examples
• Updated Docs etc
• closes Nuclei Go SDK v3 #4054
• docs about library at
  

  nuclei/v2/lib/README.md

  Lines 1 to 87 in 186a4da

  	## Using Nuclei as Library
  	Nuclei was primarily built as a CLI tool, but with increasing choice of users wanting to use nuclei as library in their own automation, we have added a simplified Library/SDK of nuclei in v3
  	### Installation
  	To add nuclei as a library to your go project, you can use the following command:
  	```bash
  	go get -u github.com/projectdiscovery/nuclei/v2/lib
  	```
  	Or add below import to your go file and let IDE handle the rest:
  	```go
  	import nuclei "github.com/projectdiscovery/nuclei/v2/lib"
  	```
  	## Basic Example of using Nuclei Library/SDK
  	```go
  	// create nuclei engine with options
  	ne, err := nuclei.NewNucleiEngine(
  	nuclei.WithTemplateFilters(nuclei.TemplateFilters{Severity: "critical"}), // run critical severity templates only
  	)
  	if err != nil {
  	panic(err)
  	}
  	// load targets and optionally probe non http/https targets
  	ne.LoadTargets([]string{"scanme.sh"}, false)
  	err = ne.ExecuteWithCallback(nil)
  	if err != nil {
  	panic(err)
  	}
  	defer ne.Close()
  	```
  	## Advanced Example of using Nuclei Library/SDK
  	For Various use cases like batching etc you might want to run nuclei in goroutines this can be done by using `nuclei.NewThreadSafeNucleiEngine`
  	```go
  	// create nuclei engine with options
  	ne, err := nuclei.NewThreadSafeNucleiEngine()
  	if err != nil{
  	panic(err)
  	}
  	// setup waitgroup to handle concurrency
  	wg := &sync.WaitGroup{}
  	// scan 1 = run dns templates on scanme.sh
  	wg.Add(1)
  	go func() {
  	defer wg.Done()
  	err = ne.ExecuteNucleiWithOpts([]string{"scanme.sh"}, nuclei.WithTemplateFilters(nuclei.TemplateFilters{ProtocolTypes: "http"}))
  	if err != nil {
  	panic(err)
  	}
  	}()
  	// scan 2 = run http templates on honey.scanme.sh
  	wg.Add(1)
  	go func() {
  	defer wg.Done()
  	err = ne.ExecuteNucleiWithOpts([]string{"honey.scanme.sh"}, nuclei.WithTemplateFilters(nuclei.TemplateFilters{ProtocolTypes: "dns"}))
  	if err != nil {
  	panic(err)
  	}
  	}()
  	// wait for all scans to finish
  	wg.Wait()
  	defer ne.Close()
  	```
  	## More Documentation
  	For complete documentation of nuclei library, please refer to [godoc](https://pkg.go.dev/github.com/projectdiscovery/nuclei/v2/lib) which contains all available options and methods.
  	### Note
  	| :exclamation: **Disclaimer** |
  	|---------------------------------|
  	| **This project is in active development**. Expect breaking changes with releases. Review the release changelog before updating. |
  	| This project was primarily built to be used as a standalone CLI tool. **Running nuclei as a service may pose security risks.** It's recommended to use with caution and additional security measures. |

Design Considerations

• SDK should be easy to get started with minimal code
• Use Option Style syntax to abstract options (we have so many options in nuclei >30 this style of syntax i.e option style is more powerful and suitable for such cases)
• Thread Safe Nuclei Execution example/support
• User should only need to have 1 import to avoid searching over multiple packages for docs

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)

[tarunKoyalwar]

View SDK docs

cd v2/lib
godoc -http=:6060 

^ this will start a local doc server with final docs which includes testable examples at http://localhost:6060/pkg/github.com/projectdiscovery/nuclei/v2/lib/

[tarunKoyalwar]

New Changes

• remove old implementation of stats and use new clistats logic
• fix issue with clustering (-dc flag was not used while clustering)
• fix stats while using SDK/library mode
• deprecated -metrics option since clistats by default starts a server for stats
• -mp now directly updates port in clistats and thus eliminating port already in use issue

New /metrics endpoint

{
  "requests": 3416,
  "errors": 955,
  "matched": 15,
  "startedAt": "2023-08-30T19:02:13.636473+05:30",
  "summary": "[0:00:27] | Templates: 6720 | Hosts: 1 | RPS: 126 | Matched: 15 | Errors: 955 | Requests: 3416/9963 (34%)\n",
  "rps": "126",
  "total": 9963,
  "templates": 6720,
  "hosts": 1,
  "duration": "0:00:27",
  "percent": "34"
}

[Mzack9999]

lgtm - nice one! This was really a must-do. The absence of consistent components initializations with reasonable defaults along with a too long boilerplate to just create an instance was definitely a usability anti-pattern.