How to cast binary field in brpc::ThriftService

Hi:
my thrift file:
```
service EchoService {
binary Echo(1:binary request);
}
```
How can i cast `brpc::ThriftFramedMessage*` to `std::string` or `butil::IOBuf`, and cast `std::string` to `brpc::ThriftFramedMessage*`? thank you!

The issue names brpc::ThriftService and brpc::ThriftFramedMessage but provides no file or test. Start by locating those APIs and their existing serialization or conversion paths, then determine whether the requested binary, string, and butil::IOBuf conversions are supported. Done means documenting the supported usage or defining a scoped implementation and test plan.

Written by the indexing model from the issue text.