feat(activitiypub): enable HTTP signatures on all ActivityPub endpoints (#7035)

- Set the right keyID and use the right signing keys for outgoing requests.
- Verify the HTTP signature of all incoming requests, except for the server actor.
- Caches keys of incoming requests for users and servers actors.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7035
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: famfo <famfo@famfo.xyz>
Co-committed-by: famfo <famfo@famfo.xyz>

modules/test/distant_federation_server_mock.go

@@ -95,7 +95,7 @@ func (mock *FederationServerMock) DistantServer(t *testing.T) *httptest.Server {
 			})
 	}
 	for _, repository := range mock.Repositories {
-		federatedRoutes.HandleFunc(fmt.Sprintf("/api/v1/activitypub/repository-id/%v/inbox/", repository.ID),
+		federatedRoutes.HandleFunc(fmt.Sprintf("/api/v1/activitypub/repository-id/%v/inbox", repository.ID),
 			func(res http.ResponseWriter, req *http.Request) {
 				if req.Method != "POST" {
 					t.Errorf("POST expected at: %q", req.URL.EscapedPath())