allow json.Marshal overriding

[qustavo]

I have a client that requires the json payload to be formatted following some specific rules.
The following PR allow SDK users to override the default json.Marshal implementation and set whatever they need.

Note: I tried implementing MarshalJSON but it does not work because json.Marshal reformats the json payload.

[subomi]

@qustavo I'm assuming this is for the webhooks payload alone. Can you give an example of the specific rules in this instance?

[qustavo]

@qustavo I'm assuming this is for the webhooks payload alone. Can you give an example of the specific rules in this instance?

Abolutely, take the following example:

package convoy

import (
	"encoding/json"
	"log"
	"testing"
)

type CreateEventRequest struct {
	EndpointID     string            `json:"endpoint_id"`
	EventType      string            `json:"event_type"`
	IdempotencyKey string            `json:"idempotency_key"`
	CustomHeaders  map[string]string `json:"custom_headers"`
	Data           json.RawMessage   `json:"data"`
}

type Model struct {
	Int    int
	String string
}

func (m *Model) MarshalJSON() ([]byte, error) {
	type Alias Model
	return json.MarshalIndent(Alias(*m), "", "  ")
}

func TestJSON(t *testing.T) {
	data, _ := json.MarshalIndent(&Model{
		Int: 42, String: "Hello",
	}, "", " ")

	ev := CreateEventRequest{
		Data: data,
	}

	bz, _ := json.Marshal(ev)
	log.Printf("%s", string(bz))
}

Will print

2024/09/23 11:26:10 {"endpoint_id":"","event_type":"","idempotency_key":"","custom_headers":null,"data":{"Int":42,"String":"Hello"}}
PASS

Regardless of my MarshalJSON implementation, json.Marshal will ignore the indentation and remove it.
In my case formatting the data payload (with indentation) is particularly important because my client uses the payload to produce a signature, sucha a signature won't match when the indentation is changed.