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>

.deadcode-out

@@ -34,9 +34,6 @@ forgejo.org/models/dbfs
 	Create
 	Rename
 
-forgejo.org/models/forgefed
-	GetFederationHost
-
 forgejo.org/models/forgejo/semver
 	GetVersion
 	SetVersionString
@@ -68,7 +65,6 @@ forgejo.org/models/user
 	DeleteUserSetting
 	GetFederatedUser
 	GetFederatedUserByUserID
-	UpdateFederatedUser
 	GetFollowersForUser
 	AddFollower
 	RemoveFollower
@@ -249,6 +245,9 @@ forgejo.org/routers/web/org
 forgejo.org/services/context
 	GetPrivateContext
 
+forgejo.org/services/federation
+	Init
+
 forgejo.org/services/repository
 	IsErrForkAlreadyExist