Use casing compliant with standard JSON serialization.

Quoting [1]:

> By default proto3 JSON printer should convert the field name to
> lowerCamelCase and use that as the JSON name. An implementation
> may provide an option to use proto field name as the JSON name
> instead.

The serde plugin does this correctly, but when the rules specify
the `preserve_proto_field_names` option, two things happen:

1. The generated serde code preserves the proto field names
2. The user of these bazel rules cannot override the choice

If, instead, rules_proto_grpc avoids specifying this flag, it can
be optionally added to the rust_prost_proto_library invocation in
user code via `options`:

    options = {
        "//rust:rust_serde_plugin": ["preserve_proto_field_names"],
    },

Thus, the better default is to leave out this flag from
rules_proto_grpc and instead allow the user to choose.

[1]: https://protobuf.dev/programming-guides/proto3/#json

rust/BUILD.bazel

@@ -51,7 +51,6 @@ proto_plugin(
     ],
     options = [
         "no_include",
-        "preserve_proto_field_names",
         "extern_path=.google.protobuf=::pbjson_types",
     ],
     output_directory = True,