refactor & enhance AP elements used (#7728)

This PR is part of https://codeberg.org/forgejo/forgejo/pulls/4767 filed by @algernon

To keep changes isolated this might be reviewed &  merge after https://codeberg.org/forgejo/forgejo/pulls/7714

Refactor existing code to have one AP struct per file.
Enhance AP structs needed by related PR

## Checklist

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### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Co-authored-by: zam <mirco.zachmann@meissa.de>
Co-authored-by: Mirco Zachmann <nostar@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7728
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/actor_repository.go

@@ -0,0 +1,52 @@
+// Copyright 2023, 2024, 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package forgefed
+
+import (
+	"fmt"
+	"strings"
+
+	"forgejo.org/modules/validation"
+)
+
+// ----------------------------- RepositoryID --------------------------------------------
+
+type RepositoryID struct {
+	ActorID
+}
+
+const (
+	repositoryIDapiPathV1       = "api/v1/activitypub/repository-id"
+	repositoryIDapiPathV1Latest = "api/activitypub/repository-id"
+)
+
+// Factory function for RepositoryID. Created struct is asserted to be valid.
+func NewRepositoryID(uri, source string) (RepositoryID, error) {
+	result, err := newActorID(uri)
+	if err != nil {
+		return RepositoryID{}, err
+	}
+	result.Source = source
+
+	// validate Person specific
+	repoID := RepositoryID{result}
+	if valid, err := validation.IsValid(repoID); !valid {
+		return RepositoryID{}, err
+	}
+
+	return repoID, nil
+}
+
+func (id RepositoryID) Validate() []string {
+	result := id.ActorID.Validate()
+	result = append(result, validation.ValidateNotEmpty(id.Source, "source")...)
+	result = append(result, validation.ValidateOneOf(id.Source, []any{"forgejo", "gitea"}, "Source")...)
+	if id.Source == "forgejo" {
+		result = append(result, validation.ValidateNotEmpty(id.Path, "path")...)
+		if strings.ToLower(id.Path) != repositoryIDapiPathV1 && strings.ToLower(id.Path) != repositoryIDapiPathV1Latest {
+			result = append(result, fmt.Sprintf("path: %q has to be a repo specific api path", id.Path))
+		}
+	}
+	return result
+}