feat(federation): validate like activities (#3494)

First step on the way to #1680

The PR will

* accept like request on the api
* validate activity in a first level

You can find

* architecture at: https://codeberg.org/meissa/forgejo/src/branch/forgejo-federated-star/docs/unsure-where-to-put/federation-architecture.md

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3494
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-committed-by: Michael Jerger <michael.jerger@meissa-gmbh.de>

routers/api/v1/activitypub/response.go

@@ -0,0 +1,35 @@
+// Copyright 2023 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package activitypub
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/activitypub"
+	"code.gitea.io/gitea/modules/forgefed"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/services/context"
+
+	ap "github.com/go-ap/activitypub"
+	"github.com/go-ap/jsonld"
+)
+
+// Respond with an ActivityStreams object
+func response(ctx *context.APIContext, v any) {
+	binary, err := jsonld.WithContext(
+		jsonld.IRI(ap.ActivityBaseURI),
+		jsonld.IRI(ap.SecurityContextURI),
+		jsonld.IRI(forgefed.ForgeFedNamespaceURI),
+	).Marshal(v)
+	if err != nil {
+		ctx.ServerError("Marshal", err)
+		return
+	}
+
+	ctx.Resp.Header().Add("Content-Type", activitypub.ActivityStreamsContentType)
+	ctx.Resp.WriteHeader(http.StatusOK)
+	if _, err = ctx.Resp.Write(binary); err != nil {
+		log.Error("write to resp err: %v", err)
+	}
+}