Support for PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION

In our code, we use `os.environ["PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION"] = "python"`
Because of this, float arrays sent with `grpcurl` are converted into double, i could be wrong on this.
In any case, adding this extra casting code is solving that, but still, it would be better if we could provide additional option to how grpcurl would handle casting implementation.

```
arr = np.array(request.user_ids)
user_ids = arr.astype(np.float)
```

The issue does not name any files or tests. Start by locating grpcurl's handling of PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION and float-array values, then determine how a command-line option could control casting behavior. Done means the requested implementation choice is configurable and float arrays are handled as intended.

Written by the indexing model from the issue text.