Add custom provider support

[ehsandeep]

Describe the solution you'd like
custom provider support to fetch subdomains from custom source which could be any public or authed URLs containing subdomain data in plain text or json and other format.

Note: regardless of the format, we just need to parse out the subdomin data.

# provider is the name of the provider
provider: custom
id: test
# urls is a list of API endpoints or resources to be accessed
urls:
  - https://api.example.com/resource1
  - https://api.example.com/resource2
# headers is a map of custom headers to be included in requests
headers:
  Authorization: $CUSTOM_AUTH_TOKEN
  Content-Type: application/json
  X-Custom-Header: $CUSTOM_HEADER

Reference - https://projectdiscovery.featurebase.app/p/custom-connect