Run processor examples, gather specifications

[lovromazgon]

Description

This PR adds the utility test function RunExample for builtin processors. The idea is that we can write runnable examples for builtin processors, which will be tested for their correctness and gathered together with the processors' specifications in a JSON file called processors.json. We can use that JSON file in our documentation to display documentation about our processors.

The function will compare the record before and after the processing and output the diff in the unified format.

Adding an example looks like this:

// 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
		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},
	})

	// 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
	//    }
	//  }
}

Closes #1289

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.

[lovromazgon]

This package doesn't really need to be reviewed, see readme.