feat: implement `attachLdSignatureForRelays` to control signing of Relayed activities (#599)

What does this PR do? (Please give us a brief description of what this PR does.)

This PR implements #599 (closed) by adding a new setting attachLdSignatureForRelays. When enabled (default), Sharkey continues its existing behavior of attaching Linked-Data signatures to all relayed activities. When disabled, LD signatures are never generated. This improves privacy at the expense of increased inbound fetches and reduced relay effectiveness.

I have tested this on my own instance and seen no unexpected issues.

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.config/ci.yml

@@ -106,7 +106,7 @@ redis:
 #   ┌───────────────────────────┐
 #───┘ MeiliSearch configuration └─────────────────────────────
 
-# You can set scope to local (default value) or global 
+# You can set scope to local (default value) or global
 # (include notes from remote).
 
 #meilisearch:
@@ -198,13 +198,15 @@ proxyRemoteFiles: true
 #   https://example.com/thumbnail.webp?thumbnail=1&url=https%3A%2F%2Fstorage.example.com%2Fpath%2Fto%2Fvideo.mp4
 #videoThumbnailGenerator: https://example.com
 
-# Sign to ActivityPub GET request (default: true)
+# Sign outgoing ActivityPub GET request (default: true)
 signToActivityPubGet: true
+# Sign outgoing ActivityPub Activities (default: true)
+attachLdSignatureForRelays: true
 # check that inbound ActivityPub GET requests are signed ("authorized fetch")
 checkActivityPubGetSignature: false
 
 # For security reasons, uploading attachments from the intranet is prohibited,
-# but exceptions can be made from the following settings. Default value is "undefined". 
+# but exceptions can be made from the following settings. Default value is "undefined".
 # Read changelog to learn more (Improvements of 12.90.0 (2021/09/04)).
 #allowedPrivateNetworks: [
 #  '127.0.0.1/32'