Un-quoted integer response values

When making a request to a gRPC method returning a message that contains an integer, the JSON structure printed to stdout wraps these numbers in double quoues.

E.g.:
```bash
> grpcurl localhost:50051 Service.Method
{
"value": "12"
}
```
The same also applies to repeated integers, in which case the output would be e.g. `{ "value": ["1", "2", "3"] }`, despite `value`, in this example, being a `repeated int64`.

Is there a reason for this behaviour? Is it possible to disable it, i.e. to tell grpcurl to format them as unqouted numbers?

The issue names no files or tests. Start by tracing grpcurl's JSON output entry point and checking the protobuf JSON rules for integer fields, then determine how an opt-out could be exposed. Done means integer and repeated-integer values can be emitted without quotes when requested, while existing output remains unchanged by default.

Written by the indexing model from the issue text.