Enhancement: generalise the validation in wrapper library

[agni-lanka]

Adding a validation wrapper Library separated from manifests. By adding this in our library we are avoiding each validation to be added by each consumer.

cloudevents/spec#2 aspects:

1. validation - IN-SCOPE
2. governance - OUT-OF-SCOPE

sdk-java issue reference#: #592
slack Thread : https://cloud-native.slack.com/archives/C9DB5ABAA/p1696404658583879?thread_ts=1695874684.630259&cid=C9DB5ABAA

[agni-lanka]

Summary / Theme / High level :
...Validation Manifest:
A JSON file (ValidationManifest.json) is the in-situ placeholder to define validation rules for CloudEvent properties.
1/allowed values; 2/range of values; 3/regex etc.

...Core CloudEvents Library:
A ValidationService class that takes the validation manifest and performs property validation based on the rules defined within.

...Wrapper Library:
Offers a CustomCloudEventValidator class that implements the CloudEventValidator interface.
This class uses the ValidationService from the core library to perform customizable property validation.

...Application Code:
Consumers of the CloudEvent w/ or w/o validation, can instantiate and use the CustomCloudEventValidator from the wrapper library to validate CloudEvent properties based on the rules defined in the validation manifest.

---

new:
ValidationRule.java
ValidationService.java
CloudEventValidator.java
CloudEventsTest.java
configuration\ValidationManifest.json

---

Consumption:
package io.cloudevents;
import io.cloudevents.CloudEvent;
import io.cloudevents.CustomCloudEventValidator;
import java.io.IOException;
public class CloudEventsTest {
public static void main(String[] args) throws IOException {
String manifestFilePath = "configuration\ValidationManifest.json";
CustomCloudEventValidator customValidator = new CustomCloudEventValidator(manifestFilePath);
// magic here
// Set CloudEvent properties
CloudEvent event = new CloudEvent();
event.setProperty1("balalah Value");
customValidator.validate(event);
}
}

[duglin]

@agni-lanka is this a "spec" (or CE-wide) issue or a java specific issue?

[agni-lanka]

@duglin - subjectively for sdk-java.

however, needs a cloudevent "spec" level for schematic/rule for extensible validations

[duglin]

@agni-lanka can you elaborate, perhaps with a small example, of what kind of spec changes (or new spec) you'd be looking for? I find it's easier for people to understand concepts when there are concrete examples.