Support client side reflection

## Feature Request

### Crates

### Motivation
The motivation comes from a use case in LLM function calling, which returns the function name and it's arguments in a json format:
```
"tool_calls": [
{
"function": {
"arguments": "{\"namespace_path\":\"***.Resource\"}",
"name": "search_file"
},
"id": "call_KS7yzQlKaa4qBMPNwKPBS8uX",
"index": 0,
"type": "function"
}
]

```
The function `search_file` is actually a gPRC method hosted in the remote server. I'm looking for ways to make generic gRPC calls without deconstructing the arguments into the required proto message struct. Actually, I don't and shouldn't care about the structure of the request and response.

### Proposal

Should support such client side reflection or ways to make generic rpc calls.

### Alternatives

a related crate might be https://crates.io/crates/prost-reflect. I hope this can be supported by tonic natively.