[Bug] When using uint64 or fixed64 types, string should be generated instead of number

[shuqingzai]

See: https://protobuf.dev/programming-guides/proto3/#json

syntax = "proto3";

import "google/api/field_behavior.proto";

message Reply {
    fixed64 id = 1 [
        (google.api.field_behavior) = REQUIRED
    ];  
   uint64 userId = 2 [
        (google.api.field_behavior) = REQUIRED
    ];  
}

[ericwenn]

This sounds great, PRs welcome!