fix: handle NaN and Infinity float values in gRPC

[ariasmn]

trafficstars

Fixes (?): https://github.com/grafana/k6/issues/3990

The problem was in this specific line:

https://github.com/grafana/k6/blob/a74d8039a2be85a7f24199f92ae1680426ee44d7/internal/js/modules/k6/grpc/client.go#L363

This MarshalJSON implemented in Sobek uses a copy of JSON.stringify() under the hood, and per JSON RFC, neither Infinity or NaN as values are accepted as valid numbers.

However, since we rely on ProtoJSON, which does support special strings "Infinity" and "NaN", I guess that the idea would be to "normalize" them as a string format before marshaling, which is what this PR implements.