fix file input in custom vars for self contained http template

[tarunKoyalwar]

Proposed changes

• fix file input in custom variables for self contained http templates
• adds Integration test for this issue to avoid regression
• closes Payload handling issue in self-contained template in base format #3361

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]

$ go run . -v -t ~/test/var_test.yaml -V "test=/tmp/number.txt"  

                     __     _
   ____  __  _______/ /__  (_)
  / __ \/ / / / ___/ / _ \/ /
 / / / / /_/ / /__/ /  __/ /
/_/ /_/\__,_/\___/_/\___/_/   v2.8.9

		projectdiscovery.io

[INF] Using Nuclei Engine 2.8.9 (latest)
[INF] Using Nuclei Templates 9.3.8 (latest)
[INF] Templates added in last update: 656
[INF] Templates loaded for scan: 1
[VER] [basic-example] Sent HTTP request to https://example.com/1
[VER] [basic-example] Sent HTTP request to https://example.com/2
[INF] No results found. Better luck next time!