Timestamp and Duration generators render wrong number format for Java

[lujiajing1126]

In the envoyproxy protos, a large number is used in the duration validation rule,

// [#next-free-field: 8]
message DownstreamTlsContext {
// ...

  // If specified, ``session_timeout`` will change the maximum lifetime (in seconds) of the TLS session.
  // Currently this value is used as a hint for the `TLS session ticket lifetime (for TLSv1.2) <https://tools.ietf.org/html/rfc5077#section-5.6>`_.
  // Only seconds can be specified (fractional seconds are ignored).
  google.protobuf.Duration session_timeout = 6 [(validate.rules).duration = {
    lt {seconds: 4294967296}
    gte {}
  }];
}

Such use cases are also shown in the examples in the README.md in this repo.

But since the user-defined thresholds are possibly out of the bound of int32, i.e. the seconds field is int64 aka long in Java.

message Duration {
  // Signed seconds of the span of time. Must be from -315,576,000,000
  // to +315,576,000,000 inclusive. Note: these bounds are computed from:
  // 60 sec/min * 60 min/hr * 24 hr/day * 365.25 days/year * 10000 years
  int64 seconds = 1;

  // Signed fractions of a second at nanosecond resolution of the span
  // of time. Durations less than one second are represented with a 0
  // `seconds` field and a positive or negative `nanos` field. For durations
  // of one second or more, a non-zero value for the `nanos` field must be
  // of the same sign as the `seconds` field. Must be from -999,999,999
  // to +999,999,999 inclusive.
  int32 nanos = 2;
}

Compiling the proto file above will just cause exceptions, (with pgv 0.9.1)

The generated codes look like,

private final com.google.protobuf.Duration SESSION_TIMEOUT__LT = io.envoyproxy.pgv.TimestampValidation.toDuration(4294967296,0); // should be 4294967296L
private final com.google.protobuf.Duration SESSION_TIMEOUT__GTE = io.envoyproxy.pgv.TimestampValidation.toDuration(0,0);

I will submit a PR to resolve this issue.