Partially addresses #9

In my earlier comment on #9, I mentioned restructuring the core traits to be sync. After reviewing the implementation, the async internals use join_all for concurrent batch processing in set_message_signatures / verify_message_signatures, so rewriting them as sync would lose that benefit. Instead, this PR takes a simpler approach.

Add MessageSignatureReqSync and MessageSignatureResSync traits as sync wrappers for the existing async traits. Each *_sync method delegates to the async counterpart via futures::executor::block_on, so the async internals remain untouched.

Gated behind a blocking feature (default-on), which only adds futures/executor as a dependency. Note that calling *_sync methods from within an async runtime will panic — this is documented on the trait definitions.