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>

models/user/user_test.go

@@ -150,7 +150,7 @@ func TestAPActorID_APActorID(t *testing.T) {
 
 func TestKeyID(t *testing.T) {
 	user := user_model.User{ID: 1}
-	url := user.APActorKeyID()
+	url := user.KeyID()
 	expected := "https://try.gitea.io/api/v1/activitypub/user-id/1#main-key"
 	assert.Equal(t, expected, url)
 }