Implement dispatching custom error remote functions

[Bhashinee]

Purpose

Introduce support to dispatch custom error remote functions. When there is a data binding error related to a custom remote function, if the WebSocket service has a custom error remote function associated with that particular custom remote function, that error remote function will get dispatched.

Related issue: ballerina-platform/ballerina-library#6625

Examples

For example,

@websocket:ServiceConfig {dispatcherKey: "event"}
service /oncustomerror on customDispatchingLis {
    resource function get .() returns websocket:Service|websocket:Error {
        return new OnCustomErrorService();
    }
}

service class OnCustomErrorService {
    *websocket:Service;
    remote function onSubscribe(websocket:Caller caller, byte[] data) returns Error? {
        check caller->writeMessage({"event": "onMessages"});
    }

    remote function onSubscribeError(websocket:Caller caller, error err) returns Error? {
        check caller->writeMessage({"event": "onCustomError"});
    }
}

Checklist

• Linked to an issue
• Updated the specification
• Updated the changelog
• Added tests
• Checked native-image compatibility

[ayeshLK]

Shall we use optionals here rather than using null ?

[ayeshLK]

Related to this comment [1]

[1] - https://github.com/ballerina-platform/module-ballerina-websocket/pull/1356/files#r1656468643

[Bhashinee]

Fixed in affa6f8

[github-actions]

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