Proposal for Custom JSON Encoder/Decoder

### Description

I am proposing the addition of a custom JSON encoder/decoder feature to the gqlgen library. This feature aims to provide an alternative for users who are not satisfied with the standard encoding/json library in gqlgen. The implementation of this feature would allow users to have a faster alternative for encoding and decoding JSON data, specifically during request processing.

### Rationale

The primary motivation behind introducing this custom JSON encoder/decoder is to enhance the performance and flexibility of gqlgen for users who require improved JSON encoding and decoding capabilities. By offering the option to replace the default encoding/json library, users can achieve better performance and tailored JSON processing for their specific use cases.

It's important to note that this custom JSON encoder/decoder would only be utilized during request processing. The existing usage of encoding/json for internal components within the library would remain unchanged to ensure stability and maintainability.

#### Performance Improvements

Several third-party libraries offer faster JSON encoding and decoding capabilities compared to encoding/json. Some of these libraries include:

- [goccy/go-json](https://github.com/goccy/go-json)
- [bytedance/sonic](https://github.com/bytedance/sonic)
- [segmentio/encoding](https://github.com/segmentio/encoding)
- [mailru/easyjson](https://github.com/mailru/easyjson)
- [minio/simdjson-go](https://github.com/minio/simdjson-go)
- [wI2L/jettison](https://github.com/wI2L/jettison)

By incorporating a customizable JSON encoder/decoder, gqlgen users would have the opportunity to leverage these third-party libraries to achieve significant performance enhancements for their GraphQL operations.

### Implementation

I propose to take the responsibility of implementing this custom JSON encoder/decoder feature myself. This would involve integrating the chosen third-party library, ensuring compatibility with gqlgen's architecture, and providing users with clear documentation on how to enable and use the custom encoder/decoder for their GraphQL operations.

I believe that this addition would greatly contribute to the overall flexibility and performance of gqlgen, catering to users with diverse needs in JSON processing.

Thank you for considering this proposal. I look forward to discussing the implementation details and potential integration of this feature into the gqlgen library.