feat(server::grpc): wire Shamir committee bootstrap and unseal proto messages

Adds DeclareCommittee and ContributePassphrase variants to bootstrap.proto,
ContributePassphrase to unseal.proto, and AwaitingContributions result codes
to both. Implements corresponding inbound converters and outbound reply
mappings. VaultGate handlers delegate to VaultCoordinator.

protobufs/operator/vault/unseal.proto

@@ -15,17 +15,23 @@ message UnsealEncryptedKey {
   bytes associated_data = 3;
 }
 
+message ContributePassphrase {
+  bytes passphrase = 1;
+}
+
 enum UnsealResult {
   UNSEAL_RESULT_UNSPECIFIED = 0;
   UNSEAL_RESULT_SUCCESS = 1;
   UNSEAL_RESULT_INVALID_KEY = 2;
   UNSEAL_RESULT_UNBOOTSTRAPPED = 3;
+  UNSEAL_RESULT_AWAITING_CONTRIBUTIONS = 4;
 }
 
 message Request {
   oneof payload {
     UnsealStart start = 1;
     UnsealEncryptedKey encrypted_key = 2;
+    ContributePassphrase contribute_passphrase = 3;
   }
 }