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/bootstrap.proto

@@ -8,15 +8,28 @@ message BootstrapEncryptedKey {
   bytes associated_data = 3;
 }
 
+message DeclareCommittee {
+  uint32 count = 1;
+}
+
+message ContributePassphrase {
+  bytes passphrase = 1;
+}
+
 enum BootstrapResult {
   BOOTSTRAP_RESULT_UNSPECIFIED = 0;
   BOOTSTRAP_RESULT_SUCCESS = 1;
   BOOTSTRAP_RESULT_ALREADY_BOOTSTRAPPED = 2;
   BOOTSTRAP_RESULT_INVALID_KEY = 3;
+  BOOTSTRAP_RESULT_AWAITING_CONTRIBUTIONS = 4;
 }
 
 message Request {
-  BootstrapEncryptedKey encrypted_key = 2;
+  oneof payload {
+    BootstrapEncryptedKey encrypted_key = 2;
+    DeclareCommittee declare_committee = 3;
+    ContributePassphrase contribute_passphrase = 4;
+  }
 }
 
 message Response {

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;
   }
 }