BP: Display processor example raw data as string

[lovromazgon]

Description

This change ensures that processor examples marshal raw data as a regular string (not base64 encoded). Examples are easier to understand this way.

If an example changes some raw data it now looks like this (notice "after": "foo"):

// Simulates a processor that will be tested.
func newTestProcessor() sdk.Processor {
	return sdk.NewProcessorFunc(sdk.Specification{
		Name:        "test",
		Version:     "v0.1.0",
		Description: "Test processor",
	}, func(ctx context.Context, rec opencdc.Record) (opencdc.Record, error) {
		rec.Operation = opencdc.OperationUpdate
		rec.Payload.After = opencdc.RawData("foo")
		return rec, nil
	})
}

func ExampleTestProcessor_1() {
	p := newTestProcessor()

	RunExample(p, example{
		Description: `When the processor receives a record with operation "create", ` +
			`it will transform the operation into "update".`,
		Config: map[string]string{},
		Have:   opencdc.Record{Operation: opencdc.OperationCreate},
		Want:   sdk.SingleRecord{Operation: opencdc.OperationUpdate, Payload: opencdc.Change{After: opencdc.RawData("foo")}},
	})

	// Output:
	// processor transformed record:
	// --- before
	// +++ after
	// @@ -1,10 +1,10 @@
	//  {
	//    "position": null,
	// -  "operation": "create",
	// +  "operation": "update",
	//    "metadata": null,
	//    "key": null,
	//    "payload": {
	//      "before": null,
	// -    "after": null
	// +    "after": "foo"
	//    }
	//  }
}

Depends on ConduitIO/conduit-commons#22.

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.