readonly support for the parameters of remote methods

[ayeshLK]

Purpose

$subject

Related to #2604

Examples

With this change developers could use reaonly parameters for remote methods in websub:SubscriberService:

@SubscriberServiceConfig {
    target: "https://discover.hub/discovery",
    leaseSeconds: 36000
}
service /subscriber on subListener {
    isolated remote function onSubscriptionValidationDenied(readonly & websub:SubscriptionDeniedError msg) returns websub:Acknowledgement? {
        // implement logic here
        return websub:ACKNOWLEDGEMENT;
    }

    remote function onUnsubscriptionVerification(readonly & websub:UnsubscriptionVerification msg)
                    returns websub:UnsubscriptionVerificationSuccess|websub:UnsubscriptionVerificationError {
        if (msg.hubTopic == "test1") {
            return websub:UNSUBSCRIPTION_VERIFICATION_ERROR;
        } else {
            return websub:UNSUBSCRIPTION_VERIFICATION_SUCCESS;
        }
    }

    isolated remote function onSubscriptionVerification(readonly & websub:SubscriptionVerification msg)
                        returns websub:SubscriptionVerificationSuccess|websub:SubscriptionVerificationError {
        if (msg.hubTopic == "test1") {
            return websub:SUBSCRIPTION_VERIFICATION_ERROR;
        } else {
            return websub:SUBSCRIPTION_VERIFICATION_SUCCESS;
        }
    }

    isolated remote function onEventNotification(readonly & websub:ContentDistributionMessage event)
                        returns websub:Acknowledgement|websub:SubscriptionDeletedError? {
        // implement logic here
        return websub:ACKNOWLEDGEMENT;
    }
}

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #500 (57fac57) into master (60e38ea) will increase coverage by 2.48%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
+ Coverage   80.28%   82.77%   +2.48%     
==========================================
  Files          16       16              
  Lines         695      714      +19     
  Branches      364      370       +6     
==========================================
+ Hits          558      591      +33     
+ Misses        131      115      -16     
- Partials        6        8       +2     

Impacted Files	Coverage Δ
...erina/stdlib/websub/NativeHttpToWebsubAdaptor.java	88.00% <89.47%> (+7.35%)	⬆️
ballerina/http_service.bal	91.66% <0.00%> (+0.92%)	⬆️
ballerina/commons.bal	100.00% <0.00%> (+4.16%)	⬆️
ballerina/native.bal	100.00% <0.00%> (+11.11%)	⬆️
ballerina/request_processor.bal	86.04% <0.00%> (+12.79%)	⬆️

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