feat(frontier): add SetGroupMemberRole RPC (#485)

Introduces a new RPC to change an existing group member's role, mirroring
SetOrganizationMemberRole. Supports user (and future principal types) with
an explicit role, replacing the implicit role assignment in AddGroupUsers.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: whoAbhishekSah

raystack/frontier/v1beta1/frontier.proto

@@ -93,6 +93,8 @@ service FrontierService {
 
   rpc RemoveGroupUser(RemoveGroupUserRequest) returns (RemoveGroupUserResponse) {}
 
+  rpc SetGroupMemberRole(SetGroupMemberRoleRequest) returns (SetGroupMemberRoleResponse) {}
+
   rpc EnableGroup(EnableGroupRequest) returns (EnableGroupResponse) {}
 
   rpc DisableGroup(DisableGroupRequest) returns (DisableGroupResponse) {}
@@ -2176,6 +2178,16 @@ message RemoveGroupUserRequest {
 
 message RemoveGroupUserResponse {}
 
+message SetGroupMemberRoleRequest {
+  string group_id = 1 [(buf.validate.field).string.uuid = true];
+  string org_id = 2 [(buf.validate.field).string.uuid = true];
+  string principal_id = 3 [(buf.validate.field).string.uuid = true];
+  string principal_type = 4 [(buf.validate.field).string.min_len = 1];
+  string role_id = 5 [(buf.validate.field).string.uuid = true];
+}
+
+message SetGroupMemberRoleResponse {}
+
 message DeleteRelationRequest {
   reserved 3;