Add GitHub/AvoidObjectSendWithDynamicMethod cop

[cheshire137]

This adds a new cop to detect when use of Object#send, #public_send, or __send__. At first I had the Rubocop flagging every use of a send method, but I scaled that back so the cop is less strict. If the method name being sent is constant, like foo.send(:bar), the cop doesn't complain. The cop also allows for partially dynamic method names, as long as some of the method being sent is constant.

Related:

• https://www.rubydoc.info/gems/gitlab-styles/RuboCop/Cop/GitlabSecurity/PublicSend - Gitlab's cop guards against any use of #send methods, even if the method being sent isn't variable, so it's more restrictive than the cop this branch adds.