Improve: optimize graphql runtime

### What happened?

The `transport.Post` implementation in the `Do` method has potential for optimization. Specifically, two areas could improve performance:
1. The `Do` method frequently converts `[]byte` to `string`, which incurs additional allocations.
2. `graphql.RawParams` could benefit from a `sync.Pool` to reduce allocations and reuse memory.

### What did you expect?

Optimizing memory allocation in the `Do` method should lead to better overall performance for the gqlgen server, especially for high-throughput applications.

### Minimal graphql.schema and models to reproduce

I tested using the gqlgen initial schema and an empty slice return for the `Todos` query, observing allocation patterns during benchmarking.

### Versions
- gqlgen version: `v0.17.55` (via `go run github.com/99designs/gqlgen version`)
- Go version: `go1.23.2 darwin/amd64`

### Pull Request

https://github.com/99designs/gqlgen/pull/3373