[all] Add stub for new Endpoint kernel object

lib/mammoth/include/mammoth/endpoint_client.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <glacier/container/pair.h>
+#include <glacier/status/error_or.h>
+#include <zcall.h>
+#include <ztypes.h>
+
+class EndpointClient {
+ public:
+  static EndpointClient AdoptEndpoint(z_cap_t cap);
+
+  template <typename Req, typename Resp>
+  glcr::ErrorOr<glcr::Pair<Resp, z_cap_t>> CallEndpoint(const Req& req);
+
+ private:
+  EndpointClient(uint64_t cap) : cap_(cap) {}
+  z_cap_t cap_;
+};
+
+template <typename Req, typename Resp>
+glcr::ErrorOr<glcr::Pair<Resp, z_cap_t>> EndpointClient::CallEndpoint(
+    const Req& req) {
+  uint64_t reply_port_cap;
+  RET_ERR(ZEndpointSend(cap_, sizeof(Req), &req, &reply_port_cap));
+
+  Resp resp;
+  z_cap_t cap = 0;
+  uint64_t num_caps = 1;
+  uint64_t num_bytes = sizeof(Resp);
+  RET_ERR(ZReplyPortRecv(reply_port_cap, &num_bytes, &resp, &num_caps, &cap));
+
+  if (num_bytes != sizeof(resp) || num_caps != 1) {
+    return glcr::FAILED_PRECONDITION;
+  }
+
+  return glcr::Pair{resp, cap};
+}