Exposing embedded api for settings control in CLI modality

[Mzack9999]

Proposed changes

Exposing embedded http server allowing concurrency/speed control. Follow up of #4986

# start with nuclei .... -hae localhost:9000
# get speed
$ curl http://localhost:9000/api/concurrency | jq
{
  "bulk_size": 1,
  "threads": 1,
  "rate_limit": 1,
  "rate_limit_duration": "1s",
  "payload_concurrency": 25,
  "probe_concurrency": 50,
  "javascript_concurrency": 100
}

# update speed
$ curl -X PUT http://localhost:9000/api/concurrency -H "Content-Type: application/json" -d '{"threads": 25, "payload_concurrency": 25, "rate_limit":500, "bulk_size":25, "rate_limit_duration":"1s"}'

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)