Improved signature handling & instance actor (#8275)

This PR is part of https://codeberg.org/forgejo/forgejo/pulls/4767

It improves the signature handling:
1. move logic to a service (might be used from other services as well)
2. make a clear difference between ` ReqHTTPUserSignature` and `ReqHTTPUserOrInstanceSignature`
3. improve test ability (activitypub/client & distant_federation_server_mock

Adjust instance actor
1. name &
2. webfinger

## Strategy for next PRs is

Integration tests are in the driving seat.

I will step by step add integration tests form original PR and add code required by the integration test changes.

## Meta

Proposal howto process large PRs can be discussed here: https://codeberg.org/forgejo-contrib/federation/pulls/37

Current state with rendered diagrams can be found here: https://codeberg.org/meissa/federation/src/branch/merge-large-pr/doc/merge-large-pr.md

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8275
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/structs/activitypub.go

@@ -1,4 +1,5 @@
 // Copyright 2022 The Gitea Authors. All rights reserved.
+// Copyright 2024 The Forgejo Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
 package structs
@@ -7,3 +8,15 @@ package structs
 type ActivityPub struct {
 	Context string `json:"@context"`
 }
+
+type APRemoteFollowOption struct {
+	Target string `json:"target"`
+}
+
+type APPersonFollowItem struct {
+	ActorID string `json:"actor_id"`
+	Note    string `json:"note"`
+
+	OriginalURL  string `json:"original_url"`
+	OriginalItem string `json:"original_item"`
+}