Add DNS provider for Core-Networks

[ldez]

• add a description to your PR
• have a homogeneous design with the other providers
• add tests (units)
• add tests ("live")
• add a provider descriptor
• generate CLI help, documentation, and readme.
• be able to do: (and put the output of this command to a comment)

  make build
  rm -rf .lego
  
  HOSTINGNL_API_KEY="xxxxxxxxxxxxxxxxxxxxx" \
  ./dist/lego -m your@email.com --dns corenetworks -d *.example.com -d example.com -s https://acme-staging-v02.api.letsencrypt.org/directory run

  Note the wildcard domain is important.
• pass the linter
• do go mod tidy

Ping @renne, can you run the command (with your domain, email, and credentials)?

Closes #1989

[ldez]

@renne do you need help to run the command?