core/src/signed_envelope: Don't take ownership of key in SignedEnvelope::new

[mxinden]

This boils down to the patch below:

diff --git a/core/src/signed_envelope.rs b/core/src/signed_envelope.rs
index 73efcc99..cdf8e9f3 100644
--- a/core/src/signed_envelope.rs
+++ b/core/src/signed_envelope.rs
@@ -19,7 +19,7 @@ pub struct SignedEnvelope {
 impl SignedEnvelope {
     /// Constructs a new [`SignedEnvelope`].
     pub fn new(
-        key: Keypair,
+        key: &Keypair,
         domain_separation: String,
         payload_type: Vec<u8>,
         payload: Vec<u8>,

[laurentsenta]

Happy take this, thanks for sharing the first step!