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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Did you actually confirm that instances that receive Announces from relays of activities generated without ld signatures, fetch the activities from the source?

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