Re-visit API for Creating Endpoints

[Neverlord]

Currently, users create an endpoint directly by calling its constructor. However, creating an endpoint can fail for several reasons. The same is true for configuration.

In https://github.com/zeek/broker/pull/81#issuecomment-575914374, we've discussed this straw-man proposal to improve error signaling to the user by going through factory functions:

int main(int argc, char** argv) {
  if (auto ep = endpoint::make(argc, argv); // passing (argc, argv) is optional
      not ep) {
    // oops, configuration error
    auto& err = ep.error();
    // ...
  } else {
    // ok, endpoint up and running
  }
}

// With explicitly using a configuration.
int main(int argc, char** argv) {
  configuration conf;
  // ... tweak default settings ...
  if (auto err = conf.init(argc, argv)) {
    // deal with error explicitly, this step is optional and otherwise performed by endoint::run
  }
  if (auto ep = endpoint::make(std::move(conf);
      not ep) {
    // ...
  } else {
    // ...
  }
}