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>

modules/forgefed/activity.go

@@ -0,0 +1,65 @@
+// Copyright 2023, 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package forgefed
+
+import (
+	"time"
+
+	"code.gitea.io/gitea/modules/validation"
+
+	ap "github.com/go-ap/activitypub"
+)
+
+// ForgeLike activity data type
+// swagger:model
+type ForgeLike struct {
+	// swagger:ignore
+	ap.Activity
+}
+
+func NewForgeLike(actorIRI, objectIRI string, startTime time.Time) (ForgeLike, error) {
+	result := ForgeLike{}
+	result.Type = ap.LikeType
+	result.Actor = ap.IRI(actorIRI)   // Thats us, a User
+	result.Object = ap.IRI(objectIRI) // Thats them, a Repository
+	result.StartTime = startTime
+	if valid, err := validation.IsValid(result); !valid {
+		return ForgeLike{}, err
+	}
+	return result, nil
+}
+
+func (like ForgeLike) MarshalJSON() ([]byte, error) {
+	return like.Activity.MarshalJSON()
+}
+
+func (like *ForgeLike) UnmarshalJSON(data []byte) error {
+	return like.Activity.UnmarshalJSON(data)
+}
+
+func (like ForgeLike) IsNewer(compareTo time.Time) bool {
+	return like.StartTime.After(compareTo)
+}
+
+func (like ForgeLike) Validate() []string {
+	var result []string
+	result = append(result, validation.ValidateNotEmpty(string(like.Type), "type")...)
+	result = append(result, validation.ValidateOneOf(string(like.Type), []any{"Like"}, "type")...)
+	if like.Actor == nil {
+		result = append(result, "Actor should not be nil.")
+	} else {
+		result = append(result, validation.ValidateNotEmpty(like.Actor.GetID().String(), "actor")...)
+	}
+	if like.Object == nil {
+		result = append(result, "Object should not be nil.")
+	} else {
+		result = append(result, validation.ValidateNotEmpty(like.Object.GetID().String(), "object")...)
+	}
+	result = append(result, validation.ValidateNotEmpty(like.StartTime.String(), "startTime")...)
+	if like.StartTime.IsZero() {
+		result = append(result, "StartTime was invalid.")
+	}
+
+	return result
+}