Motivation:

The GRPCHTTP2Core module needs a DNS resolver, as currently, IP addresses have to be passed in manually.

Modifications:

- Add a new type `SimpleAsyncDNSResolver` with a method `resolve(host:port:)` that calls the `getaddrinfo` `libc` function to resolve a hostname and port number to a list of IP addresses and port numbers. `resolve(host:port:)` is non-blocking and asynchronous. As calls to `getaddrinfo` are blocking, `resolve(host:port)` executes `getaddrinfo` in a `DispatchQueue` and uses a `CheckedContinuation` to interface with the execution.

Result:

The GRPCHTTP2Core module will have a DNS resolver.