rpc: add PeerInfo (#24255)

This replaces the sketchy and undocumented string context keys for HTTP requests
with a defined interface. Using string keys with context is discouraged because
they may clash with keys created by other packages.

We added these keys to make connection metadata available in the signer, so this
change also updates signer/core to use the new PeerInfo API.

rpc/server_test.go

@@ -45,7 +45,7 @@ func TestServerRegisterName(t *testing.T) {
 		t.Fatalf("Expected service calc to be registered")
 	}
 
-	wantCallbacks := 9
+	wantCallbacks := 10
 	if len(svc.callbacks) != wantCallbacks {
 		t.Errorf("Expected %d callbacks for service 'service', got %d", wantCallbacks, len(svc.callbacks))
 	}