Added JSON format support for templates

[CodFrm]

Proposed changes

fix some json deserialization issues

{
    "id": "self-signed-ssl",
    "info": {
        "name": "Self Signed SSL Certificate",
        "author": "righettod,pdteam",
        "severity": "low",
        "reference": [
            "https://www.rapid7.com/db/vulnerabilities/ssl-self-signed-certificate/"
        ],
        "description": "self-signed certificates are public key certificates that are not issued by a certificate authority. These self-signed\ncertificates are easy to make and do not cost money. However, they do not provide any trust value.\n",
        "remediation": "Purchase or generate a proper SSL certificate for this service.\n",
        "tags": "ssl"
    },
    "ssl": [
        {
            "address": "{{Host}}:{{Port}}",
            "matchers": [
                {
                    "type": "dsl",
                    "dsl": [
                        "self_signed == true"
                    ]
                }
            ]
        }
    ]
}

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]

@CodFrm , I fixed a minor bug and added some missing tags along with a unit test .
currently Unit Test Fails because there is a marshal/unmarshal issue with enum types.

Problem/Reason

• In some enums ex: severity , signature we use iota and iota+1 for initialisation. this causes empty string to be marshalled and other similar issue causes due to inconsistency

$ go test -v ./... -run "TestTemplateStruct"                                                                                                                                                                                                                                     
=== RUN   TestTemplateStruct
    templates_test.go:27: 
                Error Trace:    /Users/tarun/reviews/nuclei/v2/pkg/templates/templates_test.go:27
                Error:          Expected nil, but got: invalid attack type: 
                Test:           TestTemplateStruct
                Messages:       failed to unmarshal json template
--- FAIL: TestTemplateStruct (0.00s)

[tarunKoyalwar]

Note:
This is a JSON Marshal issue and not Unmarshal.
@CodFrm are you only unmarshalling JSON templates ??

[CodFrm]

@tarunKoyalwar
I hope that json and yaml can be converted to each other, but it seems to be difficult now.

while parsing json templates is ok

func TestTemplateStruct(t *testing.T) {
	bin := `{
  "id": "valid-gmail-checker",
  "info": {
    "name": "Valid Google Mail Checker",
    "author": "dievus,dwisiswant0",
    "severity": "info",
    "reference": [
      "https://github.com/dievus/geeMailUserFinder"
    ]
  },
  "self-contained": true,
  "requests": [
    {
      "method": "HEAD",
      "path": [
        "https://mail.google.com/mail/gxlu?email={{email}}"
      ],
      "matchers": [
        {
          "type": "word",
          "part": "header",
          "words": [
            "COMPASS"
          ]
        }
      ]
    }
  ]
}`
	var jsonTemplate Template
	err := json.Unmarshal([]byte(bin), &jsonTemplate)
	require.Nil(t, err, "failed to unmarshal json template")
}

[CodFrm]

Added the ability to judge an empty string when deserializing, and do nothing if it is empty

	s := strings.Trim(string(data), `"`)
	if s == "" {
		return nil
	}

[tarunKoyalwar]

@CodFrm , nice catch on the empty string deserialization issue , it seems like we almost have full support to JSON templates. currently it does not omit tags we are looking into a custom enum type so that will be fixed in near future.

[tarunKoyalwar]

lgtm !

[tarunKoyalwar]

@CodFrm by the way is there a specific usecase for this json support ? maybe we can add some helper functions in a followup issue/PR ??

[CodFrm]

@tarunKoyalwar We can implement a Network Catalog to read templates from remote servers and JSON is better for network transportation.

[Mzack9999]

lgtm - xxxTypeHolder and iota offset will be reworked and simplified in the future