feat: Allow workspace resources to attach multiple agents (#942)

This enables a "kubernetes_pod" to attach multiple agents that
could be for multiple services. Each agent is required to have
a unique name, so SSH syntax is:

`coder ssh <workspace>.<agent>`

A resource can have zero agents too, they aren't required.

cli/ssh_test.go

@@ -40,12 +40,12 @@ func TestSSH(t *testing.T) {
 						Resources: []*proto.Resource{{
 							Name: "dev",
 							Type: "google_compute_instance",
-							Agent: &proto.Agent{
+							Agents: []*proto.Agent{{
 								Id: uuid.NewString(),
 								Auth: &proto.Agent_Token{
 									Token: agentToken,
 								},
-							},
+							}},
 						}},
 					},
 				},
@@ -98,12 +98,12 @@ func TestSSH(t *testing.T) {
 						Resources: []*proto.Resource{{
 							Name: "dev",
 							Type: "google_compute_instance",
-							Agent: &proto.Agent{
+							Agents: []*proto.Agent{{
 								Id: uuid.NewString(),
 								Auth: &proto.Agent_Token{
 									Token: agentToken,
 								},
-							},
+							}},
 						}},
 					},
 				},