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/validation/validatable_test.go

@@ -0,0 +1,65 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package validation
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/timeutil"
+)
+
+type Sut struct {
+	valid bool
+}
+
+func (sut Sut) Validate() []string {
+	if sut.valid {
+		return []string{}
+	}
+	return []string{"invalid"}
+}
+
+func Test_IsValid(t *testing.T) {
+	sut := Sut{valid: true}
+	if res, _ := IsValid(sut); !res {
+		t.Errorf("sut expected to be valid: %v\n", sut.Validate())
+	}
+	sut = Sut{valid: false}
+	if res, _ := IsValid(sut); res {
+		t.Errorf("sut expected to be invalid: %v\n", sut.Validate())
+	}
+}
+
+func Test_ValidateNotEmpty_ForString(t *testing.T) {
+	sut := ""
+	if len(ValidateNotEmpty(sut, "dummyField")) == 0 {
+		t.Errorf("sut should be invalid")
+	}
+	sut = "not empty"
+	if res := ValidateNotEmpty(sut, "dummyField"); len(res) > 0 {
+		t.Errorf("sut should be valid but was %q", res)
+	}
+}
+
+func Test_ValidateNotEmpty_ForTimestamp(t *testing.T) {
+	sut := timeutil.TimeStamp(0)
+	if res := ValidateNotEmpty(sut, "dummyField"); len(res) == 0 {
+		t.Errorf("sut should be invalid")
+	}
+	sut = timeutil.TimeStampNow()
+	if res := ValidateNotEmpty(sut, "dummyField"); len(res) > 0 {
+		t.Errorf("sut should be valid but was %q", res)
+	}
+}
+
+func Test_ValidateMaxLen(t *testing.T) {
+	sut := "0123456789"
+	if len(ValidateMaxLen(sut, 9, "dummyField")) == 0 {
+		t.Errorf("sut should be invalid")
+	}
+	sut = "0123456789"
+	if res := ValidateMaxLen(sut, 11, "dummyField"); len(res) > 0 {
+		t.Errorf("sut should be valid but was %q", res)
+	}
+}