Implement DNS resolver

[clintonpi]

Motivation:

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

Modifications:

• Add a new type DNSResolver with a method resolve(host:port:) that calls the getaddrinfo libc function to resolve a hostname and port number to a list of socket addresses. resolve(host:port:) is non-blocking and asynchronous.

Result:

The GRPCHTTP2Core module will have a DNS resolver.

---

This PR re-opens grpc/grpc-swift#2060.

[glbrntt]

It looks like the CI is failing because some symbols are missing, I think we need to add a C shim for Linux to get hold of these, let me do that in a separate PR.

[glbrntt]

I think these should be private, not sure it makes sense for types from lib C to make it beyond the scope of this file.

[glbrntt]

This can be a paremeterized test, can be something like this:

Suggested change

	@Test("Resolve hostname")
	func resolve() async throws {
	// Note: This test checks the IPv4 and IPv6 addresses separately (instead of
	// `DNSResolver.resolve(host: "localhost", port: 80)`) because the ordering of the resulting
	// list containing both IP address versions can be different.
	let expectedIPv4Result: [SocketAddress] = [.ipv4(host: "127.0.0.1", port: 80)]
	let expectedIPv6Result: [SocketAddress] = [.ipv6(host: "::1", port: 80)]
	let ipv4Result = try await DNSResolver.resolve(host: "127.0.0.1", port: 80)
	let ipv6Result = try await DNSResolver.resolve(host: "::1", port: 80)
	#expect(ipv4Result == expectedIPv4Result)
	#expect(ipv6Result == expectedIPv6Result)
	}
	@Test(
	"Resolve hostname",
	arguments: [
	("127.0.0.1", .ipv4(host: "127.0.0.1", port: 80)),
	("::1", .ipv6(host: "::1", port: 80)),
	] as [(String, SocketAddress)].self
	)
	func resolve(host: String, expected: SocketAddress) async throws {
	// Note: This test checks the IPv4 and IPv6 addresses separately (instead of
	// `DNSResolver.resolve(host: "localhost", port: 80)`) because the ordering of the resulting
	// list containing both IP address versions can be different.
	let result = try await DNSResolver.resolve(host: host, port: 80)
	#expect(result == expected)
	}

[glbrntt]

It looks like the CI is failing because some symbols are missing, I think we need to add a C shim for Linux to get hold of these, let me do that in a separate PR.

Okay, multiple problems here:

• EAI_ADDRFAMILY and EAI_NODATA aren't available on Linux without including <netdb.h> on and defining the _GNU_SOURCE feature test macro. We can work around this by adding a C shim library. All good.
• EAI_BADHINTS and EAI_PROTOCOL don't exist in Linux. We could declare them in a platform specific way.

I have an alternative proposal to adding a C-shim module and doing all of the platform shenanigans: just call gai_strerror with the error code instead. It's job is to provide a string description of the error, and that's ultimately what we want in our error:

  package struct GetAddrInfoError: Error, Hashable, CustomStringConvertible {
    package let description: String

    package init(code: CInt) {
      if let cString = gai_strerror(code) {
        self.description = String(cString: cString)
      } else {
        self.description = "Unknown error: \(code)"
      }
    }
  }

[glbrntt]

Let's just make this a struct which stores the errno as a CInt

[glbrntt]

We need to convert to CInt here (which is expected by ai_protocol) as IPPROTO_TCP isn't always defined as a CInt:

Suggested change

	hints.ai_protocol = IPPROTO_TCP
	hints.ai_protocol = CInt(IPPROTO_TCP)

[glbrntt]

Great, thanks @clintonpi